Inkscape is a great vector graphic software which I use a lot. For my latest project I needed to draw some timing diagrams and was sure, that there is an extension for Inkscape to support me with that.
After a quick search I found Timink. This extensions for Inkscape shows you a dialog, where you can enter a sequence of values in a line and multiple signals with one per line. Timink uses these values to generate the vector graphic. You can also setup the rise and fall time and the height and width of the signals.
Try it if you need to draw timing diagrams, it's very convenient.