Carrier concentration in the depletion region of PN Junction graph
This has nothing to do with electrical engineering and is rather just a basic graphing problem. 

You use a log scale to show a graph over many decades say 10^{-7} to 10^{2}.

On a linear graph you'd probably be able to get a scale something like 0-100. So anything less than 2 or so will look like 0 on the linear graph.