Why shouldn't one use straight lines in a normal chart or a log-chart? It always depends on context and argument that is made.
E.g. if you want to make the point that something is exponential, heck, yes, a straight line would be not appropriate in a normal chart, but very appropriate in a log-chart. If, however, you wanted to show a linear dependency of two data series, then a straight line in a log-chart would be inappropriate, and it should instead be a straight line in the normal chart.