为什么要使用loglog双对数坐标图

双对数坐标图可以放大细微的变化

In log-log graphs, both axes have a logarithmic scale.
The idea here is we use semilog or log-log graph axes so we can more easily see details for small values of y as well as large values of y.

http://www.intmath.com/exponential-logarithmic-functions/7-graphs-log-semilog.php

双对数坐标可以横跨几个数量级

A log-log graph has logarithmic coordinates along both axes. It is basically a plot of log y against log x. Log-log plots are useful for plotting scientific and technical data because they allow the data to span several orders of magnitude

http://forums.codeguru.com/showthread.php?307638-How-to-plot-Log-Log-Graph&p=1004103#post1004103