Line char - error

After reboots my 'Line Chart' nodes stop working properly i.e. they plot all new input data at the same timestamp so the chart just displays a vertical line instead of a nice graph varying over time.

Has anybody else experienced this?

I have an aggregator before the Line Chart to ensure data is only fed to the Line Chart every 67 seconds, and the Line Chart settings are all default i.e. only display the last 1 hour of data.

Does it start with an empty chart?
What is the timestamp that they show at?