One Variable, Two Variable, Three Variable, More

Here are some ideas, which are by no means exhaustive.  None of these deal with geographic data, for which map-based representations might be better.

One quantitative, one qualitative

• several frequency polygons overlayed in different colours (this is clearer than using histograms)
• several frequency polygons or histograms in separate graphs laid out with equally-marked axes
• parallel box plots (so a box plot for each value of the qualitative variable, laid out on the same axes)
• stacked histograms or stacked bar charts (so one bar for each value of the quantitative variable, with the bar split up by category), or parallel bar charts (one bar for each category at each value of the quantitative variable)

Two qualitative

• two-way grid with number of data points indicated by discs of the appropriate size
• two-way table with cells coloured on a scale according to the number of data points in that cell
• stacked bar chart
• multiple or parallel bar charts

Two quantitative, one qualitative

• scattergraph with colour or shape of points representing group
• if one quantitative variable is time, then could have a dynamic image using one of the above methods for the other two variables and have the image change over time

Three quantitative

• two variables as axes on a scattergraph, the third variable controlling the size of the point
• two variables as axes on a scattergraph, the third variable controlling the colour of the (large enough to see) point with the colours along a meaningful spectrum
• if one variable is time, then it could be a normal scattergraph which changes over time

One quantitative, two qualitative

This is much more challenging; it is hard to find a good way to display this.

• a grid of histograms or frequency polygons, one grid direction for one qualitative variable, the other for the other
• a row of stacked histograms or overlaid frequency polygons, with each histogram or frequency polygon showing one qualitative and the quantitative variable
• the quantitative variable is along the x-axis, the two qualitative ones are indicated by the colour and shape of the dot, and the dots are spread out to show an overall histogram shape; it is not clear whether to arrange the dots by colour or by shape

Four variables

• Watch the Hans Rosling video for ideas!

Hans Rosling video

• There are four quantitative variables: time (shown by movement), wealth and life expectancy (axes) and population (size of circle), and one qualitative variable: continent (colour)