class: center, middle, inverse, title-slide # Data Visualization ## Grammar of Graphics ### Andrew Irwin, <a href="mailto:a.irwin@dal.ca" class="email">a.irwin@dal.ca</a> ### Math & Stats, Dalhousie University ### 2021-01-19 (updated: 2021-01-06) --- class: middle # What is the grammar of graphics? * A way of describing a visualization * Data * Mapping from data to aesthetic features * Geometry for representing data * Scales * Facets * Statistics * Coordinates * Themes --- class: middle, center <img src="../static/ggplot-layers.png" width="80%" style="display: block; margin: auto;" /> --- class: middle ## Data We will always be using "tidy data" arranged in an array * columns are variable * rows are observations This arrangement makes it easy to use one variable for each aesthetic feature of a plot (axis position, colour, facet, text labels, ...) --- class: middle ## Aesthetic mappings We map variables to features of the graph * x axis position * y axis position * colour (outline, fill) * shape * line type * size * error bar extent * many more used less frequently, depending on the geometry Discrete and quantitative variables are handled the same way. The details of the scale and geometry will determine their interprettion. --- class: middle ## Scales * range of values on x or y axis; formatting of axis labels * mapping from values to a discrete or continuous colour scale; choice of colours * mapping from values to a shape, or line thickness, or symbol size --- class: middle ## Geometry What kind of plot do you want? .pull-left[ <img src="../static/geoms-1.png" width="80%" style="display: block; margin: auto;" /> ] .pull-right[ <img src="../static/geoms-2.png" width="80%" style="display: block; margin: auto;" /> ] --- class: middle ## Facets <img src="../static/ggplot-facet.png" width="80%" style="display: block; margin: auto;" /> --- class: middle ## Statistics & Coordinates * Do you want to perform calculations on your data before plotting? * Calculate average and standard deviation * Smooth data * Add a regression line * What coordinate system do you want to use? * Cartesian (x-y) * Fixed aspect ratio * Polar (r, `\(\theta\)`) * Map projections --- class: middle # Theme All the "non-data ink": * Titles, axis labels, tick marks * Background shading, box around plot * How and where the legend (colour, shape) is displayed * Other annotations - shading, text --- class: middle # Summary The grammar of graphics is a way of describing a visulization. `ggplot2` is a computer implementation of this idea. (Also implemented in python (plotnine), javascript (D3), julia (Gadfly).) --- class: middle # Suggested reading * Course notes --- # Self-quiz / Bonus task Aesthetics are mappings between variables in your dataset and your visualization, defined using the `aes` function. Which of the following is a possible aesthetic mapping for the `gapminder` data set? 1. `aes(country = x, pop = y)` 2. `aes(x = country, y = pop)` 3. `aes(x <- country, y <- pop)` --- # Self-quiz / Bonus task The geometry (or geom) of a plot determines how the data are displayed in your visualization. Select the true statement. 1. You can have only one geometry, such as `geom_point` per plot 2. You can use mutiple geoms per plot, for example combining both `geom_plot` and `geom_line`. --- # Self-quiz / Bonus task True or false: To change all the points in a scatterplot to red circles, you need to add variables to your data frame with values "red" and "circle" and map those variables to the colour and shape aesthetic.