The Graph software extension allows the creation of graphs by use of the <graph>...</graph> parser tag. The graphs include bar charts, pie charts, line charts, area charts, scatter plots, tag clouds, tree maps, and political maps.
Bar/line/area charts can be easily created by using Template:Graph:Chart; the documentation for this template explains its functions and use in extreme detail.
Since 2015, interactive graphs are available: mw:Extension:Graph/Interactive Graph Tutorial
See also mw:Extension:Graph/Guide.