Without buying a powermeter, the best you could probably do is a X mile time trial over the same course once a month, and only graph those data points. Graphing all of your rides average speed will probably not lead to much meaningful data. My average MPH has gone down over the last 3 months as I went from 5000ft of climbing a month to 25,000 ft of climbing a month. It certainly doesn't mean I've had backwards progress.
So, find a course with minimal or no stops, something 10-20mi, and give it all you've got every once in a while. You'll see the progress.