'Add a tick to the gradient. Return the tick.
**Arguments:**
x               Position where tick should be added.
color           Color of added tick. If color is not specified, the color will be
the color of the gradient at the specified position.
movable         Specifies whether the tick is movable with the mouse.'

def addTick(self, x, color=None, movable=True, finish=True):

    if (color is None):
        color = self.getColor(x)
    t = TickSliderItem.addTick(self, x, color=color, movable=movable)
    t.colorChangeAllowed = True
    t.removeAllowed = True
    if finish:
        self.sigGradientChangeFinished.emit(self)
    return t
