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

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

    if (color is None):
        color = QtGui.QColor(255, 255, 255)
    tick = Tick(self, [(x * self.length), 0], color, movable, self.tickSize, pen=self.tickPen)
    self.ticks[tick] = x
    tick.setParentItem(self)
    return tick
