The speed graph is just that, speed. If you move 200 pixels in 10 frames, then 400 pixels in 4 frames, there is nothing you can do other than change the position of the keyframes to get a smooth acceleration and deceleration between the 3 keyframes. It's a matter of math. Going from 0 pixels per second to 0 pixels per second between three keyframes with the maximum speed in the middle requires the distance, not the time, between the three keyframes to be the same. That's the only way to get a bell-shaped curve unless you select all keyframes, right-click, and choose Rove Over Time.