I'm presuming that you are referring to the simplified flight planning graphs that are in CAP697.
Each line represents a specific pressure altitude, you have to interpolate between the two lines if your pressure altitude differs from both of them. Eg, if the dashed line represents 37,000ft and the solid line represents 10,000ft and your pressure altitude is 23,500ft then you draw a line half way between the two lines.
It says what to do on page 26 of the CAP manual: "interpolating for altitude between the two sets of trade lines".
Btw, what does CAP stand for?
