docs: def p2c(self, p=None):
          """convert from plot to canvas coordinates.
  
          See also c2p."""
          if p is None:
              p = [0, 0]
          x = (p[0] - self.x1) * self.xscale + self.cx1
          y = (p[1] - self.y1) * self.yscale + self.cy1
          # logging.debug("p2c: ({},{}) -> ({},{})".format(p[0],p[1], x, y))
  
          return (x, y)