Above figure shows us how pH value varies with H+ concentration. H+ concentration varies from 0.1 mol dm-3 to very low values. At very low H+ concentration values, gradient of the curve is rapidly increased and cannot be read values due to that high gradient increment.
pH value is increased from 2 to 3 when concentration is decreased from 0.01 mol dm-3 to 0.001 mol dm-3
In this graph, we plot the graph by taking pH value to x axis.
At low pH values, difference of H+ ion concentration for a pH value difference is large compared to difference of H+ ion concentration in higher pH value differences.