Electron deflection tube and parabolic path

This is the electron deflection tube. The electron beam is seen in blue against the fluorescent screen because the electrons give energy to bound electrons in the material, which go up energy levels and then de-excite by emitting blue photons. The beam is bent by parallel plates above and below the beam. The Helmholtz coils are not turned on in this picture. The path is parabolic. We should be able to prove this by looking at the data. Parabolic means that y is proportional to x-squared. In other words, if I double the distance along, I should go twice as far down. The data has really surprised me. I blew the picture up really big. I first noticed that there seem to be eight divisions on the x axis but they are labelled 2, 3 ... up to 10. No zero, no 1. This suggests that they must think that the electric field starts before the plates. I read data off using the numbered scales. Here it is:

If it is parabolic, then the ratio of y to x-squared should be constant. It is.