How can I get straight lines on a UV map? How can I set straight several edges in one action?

My current workflow is to select each edge, Shift+X+0, but I'm sure there is a more efficient way to do it, as with the Loop Tools -> Relax button.

I know the UV squares has the feature to grid by shape, but what I need is to get several edges on the same axis straight without affect the angle of the edges on the other axis.

Is that possible?
Can you help?
Aside from perhaps some add-on, I'm not sure of a way to do what you want in one action. If you are open to another solution, perhaps consider the align auto function in the UV specials menu. It does basically the same thing as scaling to 0 on X or Y, but can save time as you can just use Shift+R to repeat the operator after using it. 

Just Altselect your edge, and then use W>A(Align Auto) to activate the operator, followed by Shift+R after each selection to repeat the function.