Our charts all scale to the available data within your time window - there is no way to “pin” a chart at any scale. A chart with a flat line along the bottom is in general not particularly readable and we try to present info in an obvious fashion as possible. This does mean that the axis will change if you are in a time window with a new, higher Max value than you saw before. But it also means you can see the difference between 1 and 2% on a “normal” day.
This does not make your desire any less true or valuable, though, and so I’ve reported this to our product managers as a feature request.
Some plugin authors do choose to ‘invert’ percentages for some things (although this does not make as much sense for a metric like cpu load, where “more usage” = “more interesting”. “% buffers free” type data is sometimes more usefully reported as 100 - x (and can then be alerted on when it falls ‘below’ some %age free).