I have a chart screenshot showing monthly software subscriptions as bars and advertising spend as a line. Subscriptions use the left axis; spend uses the right. The two series appear to track each other, but changing either axis range would change that visual impression.

I want Grok to describe what the chart actually supports—not jump from “these moved together” to “advertising drove subscription growth.” I only have the image, not the underlying spreadsheet.

The tricky parts

• One axis starts above zero.
• A few values are unlabeled and would need to be estimated visually.
• The chart covers only six months, with no information about promotions or pricing changes.

What would you include in a prompt for dual-axis chart interpretation? Should I ask for separate descriptions of each series before asking about any relationship between them?

I’d also like a sensible boundary between approximate chart reading and claims that require the source data. How do you keep a short chart summary useful without giving it more statistical weight than it deserves?