I want to show a stacked bar chart (single bar) that only one part of it (the dark green one that has 15%) is clickable, when clicked this section will drill down. see examples below:
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You could try to suggest the drilldown in the data, but you'll run into problems where you have a small % of the whole, but still is important to inspect: this means there's less pixel area to both suggest interactivity, and for the user to actually select and drilldown on the data.
Use the legend; it's persistent, and you can try using borders, text or a combo of both.