It’s a pixel thing, and also an issue of data set size. Graph+ shoots for around 720 intervals per plot and adjusts the grouping to achieve that. Consider your three day plot. There are more than 50,000 intervals at 5 second grouping. For even the smallest of graphs (plotting one or two items) would generate about 1.5MB of data. That would take awhile to transfer, and then only the most capable computers would be able to efficiently process that data.
And obviously, you would need a really big, very high resolution screen to get 50,000 pixels across the plot. My pretty good primary desktop monitor has about 111 horizontal pixels/inch, so a 10 inch plot might have 1,110 pixel resolution. If more than that number of datapoints are to be plotted, the plotting software and/or video driver are going to have to find a way to represent that.
For what its worth, here’s my one day dryer plot above using influxDB plot at 5 second intervals (powered by AWS servers and my 8GB desktop, output through to a 2560 monitor in full screen mode).
IoTaWatt doesn’t have the horsepower to do this, but you can upload to influxDB and get higher resolution plots depending on your display device.