Renáta Németh, Dávid Simon

ELTE

How to construct valid charts?

From a mathematical point of view every chart is valid, but of course we can say more...

*Range of the Y-axis* should be defined so as to cover the *realistically possible values* of the variable.

E.g. if the variable is the governmental educational spending, do not start with 0, and do not finish with an unrealistically high value, say, 10,000 billion $ in case of Hungary (since by using these limits values significantly different considering the level of the national economy would get too close, masking important differences).

Similarly: do not let the two endpoints of the Y-axis be the actual lowest and highest values, since there are realistically possible values out of this range (and we have seen that by narrowing the Y-range small changes are magnified)

Furthermore: presenting in the same chart additional bits of information as *points of reference* helps the reader to judge the magnitude of the trends. Turning back to a previous example: if governmental educational spending is presented on the chart, points of reference could be the annual GDP, number of students, inflation etc.