Sideview utils handling drilldown from chart legend

I have a chart that looks like this

I'm using Sideview to configure custom drilldown options. When someone clicks on a bar in the table all is well: I can get the selected series from $click.name2$ and I can the x-axis value from $click.value$. But when someone clicks on a legend entry (e.g. the blue square on the RHS in the example above) it tries to execute the drilldown with $click.value$ not set and things go wrong. How can I special case the behaviour when $click.value$ is not set?

I've tried using the switcher module, but can't get that to match on the "not set" condition...