how to insert charts from google sheets into text box
Great question! I checked and found this awesome blog post from Stefanie Sanders that should be right up your alley Using Google Docs for dynamic Canvas content.
I tested this out myself in the Assignments text box, as I am assuming that is where you would like to do this, but it could equally be done in a page as well (and anywhere the HTML editor is accessible).
You could make the first tab in your document the one with the charts and then embed the charts that way, you may need to tweak the embed code to resize the iFrame accordingly.
Alternatively if the charts do not need to be dynamic, you could also take a screenshot of them and upload the screenshot to the page.
Hope that helps.
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