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chart.js is a javascript plugin that uses the canvas element to draw the graph. the author described how to setting it up in HTML and javascript is an image but does not have a source
The grid is all about coordinate space. the author discussed how to draw a rectangle. for instance: fillRect(x, y, width, height) Draws a filled rectangle.
It was also discussed how to apply colors and different styles. One more thing is how to draw a text.
More details about the text can be obtained.