William asks: How can I make a graph that looks like this, “tweet density” style, showing time intervals? He then helpfully describes his input data: a CSV file with headers “time started, time finished, date”. Here’s a simple CSV file, tasks.csv: task,date,start,end task1,2010-03-05,09:00:00,13:00:00 task2,2010-03-06,10:00:00,15:00:00 task3,2010-03-06,11:00:00,18:00:00 task4,2010-03-07,08:00:00,1