Analyzing categorical data (Khan Academy)
If you’re struggling with how to interpret the data displayed in different charts, have a look at this unit from the Khan Academy, focusing particularly on the sections about reading various types of graphs.

How to make bad charts in 7 simple rules
For a quick overview of what not to do with charts.

The 27 worst charts of all time
For a quick overview of what not to do with charts.

A short tour of bad graphs
For a quick overview of what not to do with charts.