Building simple mathematical diagrams with free software on the quick

Some methods in constructing formulas and diagrams on the quick here.   There may be some decent software out there for free which does much the same, or otherwise pay for software that can construct the same.  Aside from free math plotters which abound on the internet, and especially free software for python (if you program in this language) like scipy, matlibplot, gnuplot, and the like.  If you were wanting to set up diagrams really fast, you could use programs like Trimble Sketchup (used to be Google sketchup) which I've found to be the quickest and easiest in setting up plot points and setting up perpendicular angles on arbitrary axis.  Generally with graph construction, I make sure the camera is set to a top view, and then plot any number of given points.  Sketchup is nice in providing a fill for a given set of inter connected points, like lines, squares, arcs, and so forth which makes for much more graphical aid constructions in math modeling in some respects, albeit for more complex curvatures and shapes, you'd probably resort to other methods, or learn, if the interface is still available, using Ruby interface which is sort of python like but different. Once having constructed your graph and having appropriated your view perspective to the appropriate level from such perspective you can obtain a 2d rendering by simply hitting the print screen.  If you are running on windows, keep in mind you'd have to open up a graphics program (like Microsoft Paint which is provided for free) and then click the paste button in Windows 7 to paste the bit map photo into your program from Window's clipboard, or alternately if you are running SketchUp from wine on a linux distro, the print screen method (in Linux Mint and Ubuntu) are the same, the output print screen goes to your home folders Pictures folder generally speaking.  Here, generally you can do some simple photo cropping and the graph is ready for import into any desired text editor or alternate drawing program.  In this case if I want to add formulation into the graph, I've used the LibreOffice Drawing application.  This is nice since you can provide graphical formulation overlays to a given imported photo, and customization in terms of the typesetting of formulation text relative the photo (or in this case the graph) can be situated generally as desired although I haven't figured out how to rotate formula object text overlays in this case.  Once the given graphics are as desired you can either export to a desired imaging format, or as I've done I simply snapshot the perspective view again, and crop the photo in a given free image editing software (Gimp in Linux, or Shotwell does the trick).  Keep in mind if you hadn't like the standard gray imaging fill textures native to drawn mesh objects in SketchUp you could import any desired texture for filling the surface if you hadn't like the graphical diagrams object textures.  Just need to add this in Sketchup before rendering your snapshot, and then follow the steps as desired for adding formula objects to the given text.  Generally speaking this works well for simple mathematical diagrams especially when dealing with those requiring models designed with orthogonal (perpendicular) lines and simple curvatures.