Changing the fonts you use for labels and annotation is one of the easiest ways to make charts more readable and less messy, which makes a chart more inviting to examine. For Datawrapper, Lisa Charlotte Muth discusses font choices and what to look for.
Can’t go wrong with another one of Muth’s clear and practical guides.
Visualize This: The FlowingData Guide to Design, Visualization, and Statistics (2nd Edition)
