I'd rather suggest you use the "single value" display mode, which also gives you the ability to color-code value ranges. e.g.
0->50% : red
else : green
If you want to stay with the chart, the problem is that 99.97 is really close to 100 on pretty much any scale. A trick could be to stack onto your avail.% the difference between its value and 100%, so to fill the (small) gap with a different color.
Or, you can simply expand the chart to a huge height and set a "1" secondary scale on the y axis. Although I really wouldn't recommend this...