Fix plot fill for one-sided plots (#361)

If all plot points are far negative, the plot widget draws its surface up to invThreshold even if that value is outside of the plot bounds. The problem started to become apparent with the change to the shared drawing context.

render/plot.go

@@ -224,13 +224,13 @@ func (p Plot) Paint(dc *gg.Context, bounds image.Rectangle, frameIdx int) {
 		}
 		if y > p.invThreshold {
 			dc.SetColor(fillColInv)
-			for ; y != p.invThreshold; y-- {
+			for ; y != p.invThreshold && y >= 0; y-- {
 				tx, ty := dc.TransformPoint(float64(x), float64(y))
 				dc.SetPixel(int(tx), int(ty))
 			}
 		} else {
 			dc.SetColor(fillCol)
-			for ; y <= p.invThreshold; y++ {
+			for ; y <= p.invThreshold && y <= p.Height; y++ {
 				tx, ty := dc.TransformPoint(float64(x), float64(y))
 				dc.SetPixel(int(tx), int(ty))
 			}