I am trying to simplify the tempo map created by the Melodyne 4 tempo drag 'n' drop integration but my attempts at freehand drawing are just making things worse.
 
Is there a way to draw straight lines from one point to another?

Use the Line draw tool.

Craig is correct; more specifically, just in case, the line tool is accessible from the Draw tool - on the Tools HUD (press 'T') or Control Bar module, press and hold the Draw tool (pen icon) and you get a pull-down menu of behaviours, of which only Freehand and Line appear to work in the Tempo view.
 
Improvements to this have been talked about before - wouldn't it be great to have the timeline be effectively like an automation lane, with points and splines? Current drawing tools work, more or less, but it can be right fiddly getting it to work the way you want. It's also been mentioned that controlling it and automating it in real time from a knob or slider would be nice... 
 
I think there's a feature request... yes, this seems to be the current/active one, please vote on it!
http://forum.cakewalk.com...-m3185476.aspx#3200780

I would also recommend that you enable snap at a resolution that matches the shortest duration between notes in the phrase  to limit the number of changes that are inserted. Continuous, 'smooth' tempo changes  (e.g. a dozen or more changes per beat) that you get when drawing with snap disabled are not found 'in nature', and can cause various playback and waveform display issues.

This, tempo drawing, is one of the places where more than one snap resolution would be extremely useful.    Snap for audio, snap for a midi note, snap for a controller, and snap for a tempo change all require a different setting.   Sure, you can keep on changing but that gets really old. 
 
Or perhaps it's not really "snap" ... it's more like, point granularity should be different in the different uses of line drawing.

williamcopper
This, tempo drawing, is one of the places where more than one snap resolution would be extremely useful.    Snap for audio, snap for a midi note, snap for a controller, and snap for a tempo change all require a different setting.   Sure, you can keep on changing but that gets really old. 
 
Or perhaps it's not really "snap" ... it's more like, point granularity should be different in the different uses of line drawing.

I'd vote for that as a feature request...