Too many indices are deleted when number of points is reduced in BarPlot

Describe the bug Too many indices are deleted when number of points is reduced in BarPlot. This leads to an index error.

To Reproduce

from kivy_garden.graph import Graph, BarPlot
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.app import App

class TestApp(App):
    plot = None
    def build(self):
        b = BoxLayout(orientation='vertical')
        graph = Graph(xmin=-50, xmax=50, ymin=-1, ymax=1)
        self.plot = BarPlot()
        graph.add_plot(self.plot)

        self.plot.points = [(0, -0.2), (10, 0.6), (15, 1.0)]
        b.add_widget(graph)
        button = Button(text='Hello world', font_size=14)
        button.bind(on_press=self.replot)
        b.add_widget(button)
        return b

    def replot(self, inst):
        self.plot.points = [(-1, 0.5)]

TestApp().run()

Expected behavior When clicking the button, the plot should show one bar.

Logs/output

Traceback (most recent call last):
   File "kivy_graph_error_in_delete.py", line 25, in <module>
     TestApp().run()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/app.py", line 855, in run
     runTouchApp()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/base.py", line 504, in runTouchApp
     EventLoop.window.mainloop()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/core/window/window_sdl2.py", line 747, in mainloop
     self._mainloop()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/core/window/window_sdl2.py", line 479, in _mainloop
     EventLoop.idle()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/base.py", line 339, in idle
     Clock.tick()
   File "/home/davidkleiven/.local/lib/python3.7/site-packages/kivy/clock.py", line 591, in tick
     self._process_events()
   File "kivy/_clock.pyx", line 384, in kivy._clock.CyClockBase._process_events
   File "kivy/_clock.pyx", line 414, in kivy._clock.CyClockBase._process_events
   File "kivy/_clock.pyx", line 412, in kivy._clock.CyClockBase._process_events
   File "kivy/_clock.pyx", line 167, in kivy._clock.ClockEvent.tick
   File "/home/davidkleiven/Documents/graph/kivy_garden/graph/__init__.py", line 1454, in draw
     vert[idx + 4] = x1
 IndexError: list assignment index out of range

Platform (please complete the following information):

OS: Linux Fedora
Python version 3.6

I have a proposed solution in PR #19. @matham Can you have one more look at this? Are you sure that the code deletes the correct amount of elements?

kivy-garden / graph

Too many indices are deleted when number of points is reduced in BarPlot #20