it is a chart that you can use to determine the performance degradation of your aircraft with takeoff distance and climb rate due to Density Altitude variations
I have attached two images, one with sample lines and the other blank for you to print out.
Let's assume that we are flying my BushBaby at MAUW. Normal takeoff distance at Morning Star (210ft alt, 15degC Red Line) is 200meters, and climb rate is 700ft/m. Draw the line from 200ft to 15.25degC and it will cross the scale at 1.0, 1.0.
now we fly to Leoriesfontein at 4000ft. Later that afternoon we want to return, but now the temp is 35deg C. Using the 4000ft and the 35deg draw the purple line. It intersects the scale at 2.2 Takeoff and 0.4 climb. So now I can see that I should expect (2.2X200)=440meters and (700X0.4)=280ft/min climb.
Based at altitude, the calculations become a little more complicated because to need first to determine your performance at sea level.
Example you are JHB based airfield at 5800ft alt. Go early one morning, measure the temp, ideally say 15 deg C, Green Line. Now get in, at MAUW and measure the performance. let's assume that you get takeoff role, 450meters and climb at 380ft/min. Now using these figures calculate your standard performance for sea level, i.e. read the scale, 2.0 takeoff and 0.45 climb. This means at sea level the plane will take off in 350m/2.0=175meters and the climb will be 380/0.45=844ft/min.
Write these figures down in your POH and use them for future calculations
The Blue line shows the same JHB airfield, but now at 40deg C. The Scale is intersected at 3.0 takeoff and 0.27 climb.
Using your sea level figures,
175m X 3 = 525 meters runway required and
844ft/min X 0.27 = 227ft/min