How to stop the X value from changing in graph editor?

[RHSeditor]

This is a weird problem to me. I'm not sure how to explain it. I shall use pictures.

I have generated a vertical beam and keyframed it to move sideways as the mask on the layer beneath reveals a logo. See the before and after photos:

Upon finishing that sideways move, I want the line to move to the bottom of the logo like so:

I have keyframed it to do so, but for some reason the "starting point" moves sideways during the second move, even though it should just be a vertical drop. The endpoint should move directly to the left and the start point should move directly downwards. Instead, it does this:

When I open the graph editor and switch to the value graph, I can see a weird bubble in the X coordinates line for the starting point, but I can't figure out how to fix it; there's no bezier handles on the value keyframes, and when I try to convert them to linear it affects them on the temporal graph as well.

I need the keyframes to remain bezier to get the effect I want on the first move, so is there something I can do to stop the X coordinate from changing during the second move?

Hope that makes sense. Thanks for any help!

[Davide Boscolo · Accepted solution]

In this case this should work:

Select your keyframes,

right click and select 'keyframe interpolation'

set spacial interpolation to Linear.

[RHSeditor]

You are a god amongst men, good sir. That worked perfectly! Thank you!