Charting Scenarios

[f2e4]

I have created a chart - attached - that is used to test various scenarios.

Sheet (Scenario Chart) = sheet with chart and inputs
Sheet (Test data) = the values I used to created the chart

I have also created boundaries within this chart:

Y axis maximum = 15.1
Y axis minimum = 7.6

X axis maximum = 6.5
X axis minimum = 2.6

My query is, even though I can move 2 independent points around the chart, how can I keep them within the boundaries.

For example:
If the 'Fuel Input' in increased, once it reaches 6.5, the value remains at 6.5 but the 'Net Thermal Output' is increased.

Note: The boundary is not a straight forward 4 sided box, it actually has 2 slanted sides, just to keep things interesting.

Any help is greatly appreciated.