Attached a gif below, a few ways to get straight lines.
First way, make a perfectly straight line automatically: Select Square tool, press Shift and click and drag on the automation line.
Second way, just enable the horizontal tool line and ballpark a line it with its help (Preferences > Editing > Tools > Show Horizontal Line)
Third way, select multiple events (with Shift-clicking) on the automation lane. Go into the Info Line, double-click to edit the value, press Ctrl + Enter. It always shows the value of the first (left-most) event and changes all the selected events to match that value. Note that the events whose value you want to change do not have to be next to each other.