HiddenChartFormatter is a Splunk module not a Sideview module. Sideview modules have params that almost universally interpret $foo$ tokens, whereas in the Splunk modules there are only a couple params in a couple modules.
1) If you're setting chartType from an upstream form element, you dont actually need to filter it through a HiddenChartFormatter. Just change the name of your Pulldown/Radio module from <param name="name">chartType</param> to <param name="name">charting.chart.chartType</param> and delete the problematic line from HiddenChartFormatter entirely.
2) If you have a complex and unusual use case, and ssuming you're on a reasonably current Sideview Utils version, you can replace HiddenChartFormatter with ValueSetter.
<module name="ValueSetter">
<param name="arg.charting.chart.chartType">$chartType$</param>
and ValueSetter can do more complex things like only overwrite upstream values if other keys are set, or set according to conditional statement testing other keys against values.
If for some reason you're still using the old LGPL version (1.3.5), you should upgrade to latest (3.1.1) Here are release notes so you can see what you're missing.
http://sideviewapps.com/apps/sideview-utils/release-notes/
and here's the licensing FAQ to reassure you that the current version is as free for internal use as the old LGPL version.
http://sideviewapps.com/apps/sideview-utils/licensing-faq/
