When scrolling back in time on a graph that populates data from a service we noticed that the last datapoint before a missing period, and the first datapoint after the same missing period were being connected with a line. The data is loaded through a service incrementally as you scale or pan the graph, and this problem only appears as you load new periods of data.
See https://www.ncdc.noaa.gov/crn/graph?config=new-temp3&dataservlet=xmldata2&station_ids=1085&last=201707270000&timezone=US/Eastern
This url will load a graph that displays correctly, until you go backwards to the 23rd. At which point the lines will connect.