Renáta Németh, Dávid Simon
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.