HTML5 Canvas and Line Width

I'm drawing line graphs on a canvas. The lines draw fine. The graph is scaled, every segment is drawn, color are ok, etc. My only problem is visually the line width varies. It's almost like the nib of a caligraphy pen. If the stroke is upward the line is thin, if the stroke is horizontal, the line is thicker.

My line thickness is constant, and my strokeStyle is set to black. I don't see any other properties of the canvas that affect such a varying line width but there must be.