This Gantt diagram was made with Jan Verhoeven's Gantt2, free from Jans Tools Collection. The program is Windows freeware.
If you want to use this, here's a tip. Start to draw the chart as the author suggests, but then save it as a .gan file. Open the .gan file with a text editor, work out what the format is, and then edit changes directly, via the text editor. (As far as I can see, this is the only way to get something to start and stop more than once along the same horizontal line.) Get Gantt2 to close the file without saving it and then to reopen it.
This is all a bit tiresome, but it beats working out how to use large and complex "project management" software.
Incidentally, the graphic here was converted to .png format by another free utility, the excellent graphics program IrfanView.