Fix for double paint of grid of x-axis

[pawelzwronek]

ctx.closePath(); makes second draw of the line. With solid line there is no difference but performance, but when the line has any pattern, this second draw makes a mess.

[coveralls]

Coverage decreased (-0.002%) to 90.193% when pulling 95180d219ea74bb80aa2cffb61fa1d13b7a7eb1d on pawelzwronek:patch-1 into e9155b2c442bf69396d5a456c885f938d38d35da on danvk:master.