tkinter histogram prints downwards - python-3.x

Question

I am trying to draw two histograms alongside one another using tkinter canvas. Everything sort of works ( looks extremely scruffy at the moment) but the histograms are drawn downwards. I have tried making the y0 value negative, but then nothing at all is drawn.
I am using two lists of numerical data, the first with 50 observations and the other with eleven observations, the scales are not the same, but it is the qualitative effect I want at the moment.
The offending code is as follows:
root = Tk()
canvas = Canvas(root, width=620, height=400, background = "salmon")
canvas.grid()
# draw x-axis lines
canvas.create_line(0,2, 500, 0, width = 2, fill = "firebrick")
canvas.create_line(505,2, 610, 0, width = 2, fill = "dark slate blue")
# draw histograms
for idx in range(len(main_counts[0])):
canvas.create_rectangle(idx*10, main_counts[0][idx], 10 +(idx*10), 0, fill = "medium sea green", outline = "firebrick")
canvas.create_text(idx*10 + 8, 40, text = idx + 1, font = ("Comic sans MS",8), fill = "firebrick")
for idx in range(len(star_counts[2])):
canvas.create_rectangle((505 + idx*10), star_counts[2][idx], (515 + (idx*10)), 0, fill = "gold", outline = "dark slate blue")
canvas.create_text(505 + idx*10 + 8, 120, text = idx + 1, font = ("Comic sans MS", 8) , fill = "dark slate blue")
root.mainloop()
I know that I am missing something quite simple and obvious to all of you, but I just can't see it or the way to make my y0 negative which will presumably solve the problem. I can also not see my x-axes, but that may be because they are occluded by the histogram bars.
Many thanks for your patience and help! Any other suggestions about formatting the graphs will be welcomed including suggestions of best font to use for small digit screen display

Answer 1

The system coordinates start in the upper-left corner so you should write something like:
main_counts =[[10, 20, 30]]
for idx in range(len(main_counts[0])):
canvas.create_rectangle(idx*10, 200 -main_counts[0][idx], 10 +(idx*10), 200, fill = "medium sea green", outline = "firebrick")
canvas.create_text(idx*10 + 8, 210, text = idx + 1, font = ("Comic sans MS",8), fill = "firebrick")
You can use enumerate for more readable code:
for idx, val in enumerate(main_counts[0]):
canvas.create_rectangle(idx*10, 200 -val, 10 +(idx*10), 200, fill = "medium sea green", outline = "firebrick")
canvas.create_text(idx*10 + 8, 210, text = idx + 1, font = ("Comic sans MS",8), fill = "firebrick")

Answer 2

In simple terms - Try to create the histogram from a point on the coordinate according to your value as your second argument in rectangle_crete() function, and then go upto the point where you want your base of the histogram should,which will me constant for all your histograms. Because Tkinter coordinates starts from (0,0) and goes from up to down.
An example code is here -
from Tkinter import *
from random import randint # for testing histogram
master = Tk()
w = Canvas(master, width=1000, height=500)
w.pack()
start_point = 70
padding_y = 450
width = 50
height = 450
list = []
for i in range(1,10):
list.append(randint(1,4))
for i in range(1,10):
end_point = start_point+width
w.create_rectangle(start_point, list[i-1]*100, end_point, height, fill="blue")
start_point += width+20
mainloop()