Prior to Pega 7.2.2 you would need to create a custom chart for this. In 7.2.2 and up you can use a chart theme, which will style all charts in your application. You can find the documentation for chart themes here. Here's what my theme file looks like with comments explaining the difference from the default theme:
Note the use of !important. This is a FusionCharts-specific feature that mirrors the CSS !important but these styles are not applied with CSS. Unfortunately the use of !important is required to override the same properties being hard coded in our chart generation code. I will open an internal bug to see about removing such hard coded properties from our code where moving them to the default chart theme makes sense.