Think of the difference between canvas and svg as the difference between Photoshop and Illustrator (or Gimp and Inkscape for you, OSS). One deals with bitmaps and another vector image.
With canvas, as you paint in a bitmap, you can easily blur, blur, burn, dodge images. But since this is a bitmap, you cannot easily draw a line, and then decide to reposition the line. You need to delete the old line and then draw a new line.
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