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In typical use cases people don’t attempt to “undo” their changes on the Canvas – it’s far easier to just clear the Canvas and start drawing all over again.
ctx.clearRect(0, 0, canvas.width, canvas.height);
// Do my drawing again
myChart.update(); // With chart.js this triggers a redraw.
In your case I think you’re making the mistake of thinking that the Canvas saves the text image and lets you alter it in subsequent drawings. Think of it like Microsoft Paint – once that text is on the image, you’re not going to be able to get rid of it easily without undo’ing.
Source:stackexchange.com