I've been doing this for years using the lap feature in my Polar 720 and (when I remember) a spreadsheet. The low-cost way would be a stopwatch (on the phone if you insist) and remember (or write it down or text it to yourself) the elapsed time and just plot it.

For something more elaborate, I'd look at MyTracks plus a spreadsheet, or the ridewithgps site if you want to bother with export & uploading. While I have not looked at it, cardiotrainer but if it does what you want in one app, that may be the way to go.