The documentation for Canvas.skew states that the units for the sx and sy parameters are in radians. However, that is incorrect, they are actually in raw "rise over run" ratio units. To see this, try a vertical skew of π/4 radians (45°). This should turn a horizontal line into a 45° diagonal line. You will see that it doesn't. However, if you instead try a vertical skew of tan(π/4), then you see the desired result.