Yes you can but not with the Line chart. You would actually use a Scatter chart for this and enable the line option. This means that you can have irregular points where the gap between points is not necessarily always the same. There's an example of a date/time chart in the demos. There are various date/time formats that are supported natively by RGraph but if you prefer you can also use the Moment.js library that's included with RGraph. There's a HOWTO document about date/time charts here that has more information.