Yes. This is because the data could not be written to the PWH while it was disconnected. That data is lost now. Once you reconnect, data will be written again from that point onward and should show up in your chart again, but you will have a gap in the data for the time when the PWH was disconnected.
In addition to Benjamin's comment, measure data (the only data that is chartable) is kept in memory for one hour, so you can create charts from that data while in memory even though it is not being persisted to the PWH when it is disconected.