Week 4: Data Visualisation and Transformation

Question 1

How many shapes can you plot as a mapping? What happens if there are more than that many factors?

Answer 1

6. Additional groups will go unplotted

Question 2

What are 2 codes to make vectors into a dataframe?
How do you specify the type?
What is a difference between the 2 codes?

Answer 2

tibble(column1= , column2= )
data.frame(column1=, column2= )

can use as.character(column1= )

data.frame automatically sets characters as factors unless you set stringsAsFactors=FALSE

Question 3

How do you give a ranking to values in a vector x?

Answer 3

can also use rank, is equivalent

min_rank(x) #gives 1 to the lowest value and so on
min_rank(desc(x)) #gives 1 to the highest value

Question 4

How do you set axis limits in 2 ways?

Answer 4

+ xlim(0,5)

+ coord_cartesian(xlim = c( , ))
this way doesn’t throw away the data outside the limits, used to zoom

Question 5

How do you define something e.g. a plot in the server and how do you refer to it in the ui in shiny?

Answer 5

# Define a plot called distplot in the server using output$distplot. 
In ui.R can call directly with distplot

Question 6

code to keep the first 200 rows and all columns of a dataframe?

Answer 6

data[1:200,]

Question 7

What is a good way to display a categorical variable vs a continuous variable?

Answer 7

boxplot
freqpoly, map by colour

Question 8

When should you use facet grid over mapping by colour?

Answer 8

As the number of points and number of categories increases, facet grid becomes better

Question 9

What are the layers of Wickhams grammar of graphics? (6)

Answer 9

• Aesthetics (position, shape, colour, …)
• Geometric objects (points, lines, bars, …)
• Scales (continuous, discrete, Cartesian coordinates, …)
• Facets (small multiples)
• Statistical transformation (identity, binning, median, …)
• Coordinate system (Cartesian, polar, parallel, …)

Question 10

How do you replace unusual values with missing values in a dataframe?

Answer 10

mutate(column = ifelse(test, value1, value2))
test is a logical vector ie column<5
value 1 is what to do when test is true
value 2 is what to do when test is false

Question 11

How do you do a cumulative sum?

Answer 11

cumsum()

Question 12

What is a high data-ink ratio?

Answer 12

more data with less ink on the page

Question 13

How do you get a freqpoly to display density instead of count?

Answer 13

geom_freqploy(aes(x=price, y=..density..))

Question 14

What are the modular arithmetic symbols?

Answer 14

%/% (integer division)
%% (remainder)

Question 15

How do you reorder boxplots by median?

Answer 15

ggplot(data) + geom_boxplot(aes(x = reorder(xvariable, yvariable, FUN = median), y = yvariable))

Question 16

What is a good way to display two categorical variables?

Answer 16

geom_count, creates a graph with different size dots at each intersection
geom_tile with fill aesthetic darker depending on combinations of each value:
data %>% count(variable1, variable2) %>% ggplot(aes(x = variable1, y = variable2)) + geom_tile(aes(fill = n))

Question 17

What is a different way of creating a matrix of plots?

Answer 17

library(GGally)
ggpairs(data, columns= c(‘column1’, ‘column2’, ‘column3’)]

Question 18

How do you remove a grouping to return to operations on ungrouped variables?

Answer 18

ungroup()

Question 19

What do aggregate functions do when there are missing values? How do you change the default?

Answer 19

Gives NA when there are missing values
Use na.rm=TRUE to remove missing values prior to calculation

Question 20

What is the general format of a ggplot code?

Answer 20

ggplot(data = ) +
geom(mapping=aes(), stat=, position=) +
+

Question 21

What are the Gestalt principles of relatedness? (7)

Answer 21

• Proximity: things that are spatially near to one another seem to be related
• Similarity: things that look alike seem to be related
• Connection: things that are visually tied to one another seem to be related
• Continuity: partially hidden objects are completed into familiar shapes
• Closure: incomplete shapes are perceived as complete
• Figure and ground: visual elements are taken to be either in the foreground or in the background
• Common fate: elements sharing a direction of movement are perceived as a unit

Question 22

What is the ranking of effectiveness for categorical attributes? (4)

Answer 22

1. spatial region
2. colour hue
3. motion
4. shape

Question 23

What happens to missing values in geom_bar and geom_histogram?

Answer 23

geom_bar creates is own bin for NAs
geom_histogram removes missing values

Question 24

What are the logical operators in R?

Answer 24

& #and
| #or
! #not

Question 25

Using facet grid, which variable should you put in the rows and which in the columns?

Answer 25

Put the variable with more levels in the columns

Question 26

What is the ranking of effectiveness for ordered attributes? (8)

Answer 26

1. position on a common scale 2. position on an unaligned scale 3. length 4. tilt/angle 5. area 6. depth 7. colour luminance/colour saturation 8. curvature/volume

Question 27

How do you arrange rows in descending order?

Answer 27

arrange(data, desc(column1))

Question 28

How do you get mean and standard deviation?

Answer 28

mean () sd()

Question 29

How do you remove the missing values from a dataframe using filter?

Answer 29

filter(!is.na(data))

Question 30

What package is ggplot2 in?

Answer 30

library(tidyverse)

Question 31

How do you change the colour to no colour?

Answer 31

colour = NA

Question 32

When using the + in ggplot, where in the line must it go?

Answer 32

Can't be at the beginning of a line. Must be at end of previous line

Question 33

What does filter do with NA values?

Answer 33

Excludes them automatically

Question 34

How do you add a new column to a dataframe and keep only the new columns?

Answer 34

transmute(data, newcolumn = …)

Question 35

How do you rename a variable?

Answer 35

rename(data, newname = variable1)

Question 36

How do you get a count of how many x\>10 in a variable? How do you get a proportion?

Answer 36

sum(x\>10) gives the number of TRUE’s in x\>10 mean(x\>10) gives the proportion of TRUEs in x\>10

Question 37

How do you select only certain columns of a dataframe?

Answer 37

select(data, variable1, variable2)

Question 38

How do you create a table with a count for each value in a variable?

Answer 38

table(variable)

Question 39

What is Steven’s Psychophysical Power Law?

Answer 39

How humans perceive sensations compared to how they actually change

Question 40

What are some useful geoms? (15)

Answer 40

geom\_point() #scatterplot geom\_density\_2d() #contour lines from x and y positions geom\_histogram() #histogram, use binwidth to change size of bins geom\_density() #density plot geom\_rug() #adds rug marks of raw data on the axes geom\_boxplot() #boxplot geom\_bar() #barplot, automatically transforms variables to counts using stat\_count geom\_col() #column graph, need x and y variables geom\_line() #line chart geom\_label(aes(x=, y=, label=)) #gives a label at the specified x and y position geom\_freqpoly() #same as histogram but uses lines instead of bars, good for overlapping data geom\_area() #area chart geom\_smooth(method=‘lm’) #draws line of best fit. se=TRUE is default and displays the standard error lines aswell geom\_abline() #adds reference line, default is from data geom\_ribbon(x, ymin, ymax) #draws line with lower and upper bounds e.g. confidence interval

Question 41

What are two ways to add jitter to a plot?

Answer 41

geom\_point(position=“jitter”) OR geom\_jitter()

Question 42

How do you use facet wrap?

Answer 42

facet\_wrap(~variable) create formula with ~, variable should be discrete

Question 43

How do you get a grouped summary (mean for this question) of a variable grouped by another variable?

Answer 43

data %\>% group\_by(variable) %\>% summarise(newname = mean(distance, na.rm=TRUE))

Question 44

How do you remove the legend?

Answer 44

show.legend = FALSE

Question 45

What is a good way to display two continuous variables? (3)

Answer 45

geom\_point (scatterplot) geom\_bin2d and geom\_hex() divide plane into 2d bins and use to colour to display how many points in each bin use boxplot and divide one continuous variable into a categorical: geom\_boxplot(aes(group = cut\_width(x, width))

Question 46

What is a good way to visualise a single continuous variable? (2)

Answer 46

``` histogram, puts continuous variables into bins density plot (smoothed histogram) ```

Question 47

How do you select columns by matching them to names contained in a string vector? (3 ways)

Answer 47

!!vector #bang-bang operator, matches to names of variables in vector all\_of(vector) any\_of(vector) #in all of, all names must be present. in any of they don't

Question 48

What does the "group" mapping do in a geom?

Answer 48

Using aes(group= ) is the same as mapping a variable onto an aesthetic, it separates based on the discrete variable but does not specify the difference with a legend on the graph

Question 49

What is a good way to visualise a single categorical variable?

Answer 49

bar chart

Question 50

How do you rearrange the columns to put certain columns at the start?

Answer 50

``` select(data, variable5, variable7, everything()) #moves variables 5 and 7 to the start but still keeps all others ```

Question 51

How do you facet a plot on a combination of 2 variables?

Answer 51

facet\_grid(variable1~variable2)

Question 52

How do you get the counts of each combination when comparing 2 categorical variables?

Answer 52

count(variable1, variable2) gives number of combinations in a table

Question 53

How do you exclude a column from a dataframe?

Answer 53

select(data, -(variable1))

Question 54

How does geom\_bar get the count for each variable? How do you override it?

Answer 54

Uses statistical transformation stat\_count() to find the count of each variable. Can override using stat = "identity" for example to use a y value instead of count

Question 55

How do you change the order of rows?

Answer 55

``` arrange(data, column1, column2) #orders by column 1 then column 2 in ascending order ```

Question 56

Can you map continuous variables to colour and shape?

Answer 56

Yes to colour, gives continuous scale. No to shape

Question 57

What are 6 coordinate systems in ggplot?

Answer 57

coord\_cartesian() #default coord\_flip() #switches the x and y axis coord\_map() #sets the aspect ratio correctly for maps coord\_quickmap() #sets the aspect ratio correctly for maps, quicker coord\_polar() #uses polar coordinates. use argument (theta=“y”) to create a pie chart. otherwise you get a bulls-eye chart coord\_fixed() #forces a specified ratio between the axes, default is 1

Question 58

How do you "flatten" a graph?

Answer 58

get the residual and plot the data against the residual

Question 59

How do you map an aesthetic to a function

Answer 59

E.g. colour = variable \<5. Gives true or false for colours

Question 60

How do you add a new column to a dataframe?

Answer 60

mutate(data, new column = …)

Question 61

How do you "straighten" a graph?

Answer 61

+ scale\_y\_log10() + scale\_x\_log10()

Question 62

What are some possible aesthetic mappings? (9)

Answer 62

colour #changes colour, size #changes size alpha #changes transparency between 0 and 1 shape #changes shape linetype #from 0-6. 0 is blank, 1 is solid, others are dotted or dashed fill #fill colour of shape or curve stroke #modify width of border. E.g. on shapes fontface #character “plain”, “bold”, “italic” “bold.italic” group #groups by a specified variable

Question 63

What is the structure of a shiny app?

Answer 63

ui

Question 64

What is Ansocombe’s quartet?

Answer 64

Facet of 4 graphs with very different data but the same line of best fit, which has the same linear regression coefficient 0.7

Question 65

How do you select column that start, with, end with, contain or match a string?

Answer 65

starts\_with(“abc”) c ends\_with(“abc”) contains(“abc”) matches(“expression”)

Question 66

What are the disadvantages of r shiny? (5)

Answer 66

- Unhelpful errors - Dependency hell - Unreliable back end - Need to run R on a server - Reactivity can be slow

Question 67

If you have different mappings in ggplot() and geom(aes()), which one will override?

Answer 67

The mapping in the geom overrides the global mapping in ggplot

Question 68

What are the comparison operators in R?

Answer 68

, \>=, !=, ==

Question 69

How do you label the axes?

Answer 69

+ labs(y= "", x= "")

Question 70

What is contained in the ui.R file in shiny?

Answer 70

provides schematic for how the application will be presented to the user. e.g. titlePanel(), sidebarLayout(), sidebarPanel(), mainPanel()

Question 71

What is contained in the server.R file in shiny?

Answer 71

contains everything which should be done in the background

Question 72

What is a violin plot?

Answer 72

Like a boxplot but shows the full density plots

Question 73

What is the order of how humans perceive sensations? (6)

Answer 73

1. electric shock 2. saturation 3. length 4. area 5. depth 6. brightness

Question 74

What do lead(x) and lag(x) do to a vector x?

Answer 74

lead(x) removes first number and adds NA to end lag(x) adds NA to beginning and removes last number

Question 75

What is a facet?

Answer 75

Facets are subplots that each display a subset of the data

Question 76

Does the aesthetic mapping go inside or outside the aes()?

Answer 76

If it gives information about a variable it goes inside the aes(). If not it must go outside the aes()

Question 77

How do you call something from the input in the server in shiny?

Answer 77

anything from the input can be called in server using input$…

Question 78

How do you select only certain rows of a dataframe based on their values?

Answer 78

filter(dataframe, value1==1, value2==4)

Question 79

How do you determine if a value is a missing value in R?

Answer 79

is.na()

Question 80

What are additional arguments to facet\_wrap? (2)

Answer 80

nrow = ncol = specifies number of rows or columns

Question 81

How do you get the max value of a variable and the location of the max value?

Answer 81

max(variable) #gives max value which.max(variable) #gives location of max value

Question 82

What are 3 distilled principles from Tufte?

Answer 82

- Maximise data-to-ink ratio - Present more data without losing interpretability - Use levels of detail

Question 83

What is reactivity in Shiny?

Answer 83

The output depends on the users input. all output must be wrapped in a renderSomething function, this watches the input and updates accordingly

Question 84

What happens when you map an aesthetic (colour) to a bar chart? What 3 ways are there to change how this looks?

Answer 84

The bars are automatically stacked with colours for each object in the variable (position = "stack") ``` position = “identity” #places each object exactly where it falls in context of graph. Will include overlapping so should use alpha position = “fill” #works like stacking but makes each set of stacked bars the same height. Makes it easier to compare proportions across groups position = “dodge” #places overlapping objects beside each other. Makes it easier to compare individual values ```

Question 85

How do you use in and between in R?

Answer 85

%in% #in between(x, left, right) #finds rows where x is between left and right

Question 86

How do you split a variable into different sections and label the sections?

Answer 86

cut(variable, breaks=(0,10,20), labels = c("", ""))

Question 87

How do you load a csv into a dataframe in R?

Answer 87

read\_csv(‘filepath’)