How to set canvas size properly in Tkinter?

Question

I am using Tkinter to visualize my data points. My problem is that I cannot make the data points appear in the center of the canvas and meanwhile the canvas is big enough.

To make the canvas look good, I wish to fix it at around 800*600 (I think the unit is pixel). So I did the following:

class DisplayParticles(Canvas):
    def __init__(self):
        # Canvas
        Canvas.__init__(self)
        self.configure(width=800, height=600)
        # Particles
        self.particle_radius = 1
        self.particle_color = 'red'
        # User
        self.user_radius = 4
        self.user_color = 'blue'
        self.ghost_color = None

However, my data to be plotted are in the unit of meter. Plus, they center around the origin (0, 0), which means that there are negative coordinates for both x and y.

Then when I plot them on the canvas I get something like this

Obviously, the data points were plotted in pixel!

I wish the canvas to be big enough on the screen and meanwhile the data are plotted in a proper scale centered at the canvas. (Place my origin (0, 0) at the canvas center)

How may I do that?

Answer 1

The visible center of the canvas can be changed by setting the scrollregion attribute. For example, if you set the scrollregion to (-400,-400, 400, 400) in a canvas that is 800 pixels wide, the coordinate 0,0 will appear in the center of the screen.

Here's an example that draws a square at 0,0, and which appears in the center of the screen:

import Tkinter as tk

class Example(tk.Frame):
    def __init__(self,*args, **kwargs):
        tk.Frame.__init__(self, *args, **kwargs)

        self.canvas = tk.Canvas(width=800, height=800)
        self.canvas.pack(side="top", fill="both", expand=True)
        self.canvas.configure(scrollregion=(-400, -400, 400, 400))

        self.canvas.create_rectangle(-10,-10,10,10, fill="red", outline="black")

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).pack(side="top", fill="both", expand=True)
    root.mainloop()

You can also use the xview_scroll and yview_scroll methods to move 0,0 into the center of the visible portion of the canvas. Instead of setting the scrollregion as in the above example, you can do this after creating your canvas:

self.canvas.xview_scroll(800, "units")
self.canvas.yview_scroll(800, "units")

This will programmatically scroll the canvas over and down 800 pixels, so that 0,0 is in the center of the screen.