Adding 40 entries per day, for 7 days, to a graph will likely result in a 'hot mess', as you put it. How critical to the operation of the machine is the 'unit' measurement, i.e. each 15 minute temperature reading? Is there a range of temperature conditions that the users need to be made aware of, i.e. 'Safe', 'Critical', etc? How would the users utilize the graph? Any additional information would be helpful.