Strictly speaking, a logarithmic scale can't plot negative values, for the simple reason that it is not mathematically correct.
Math.log(1) => 0;
Math.log(0.1) => -2.3025850929940455;
Math.log(0) => -Infinity;
Math.log(-1) => NaN;
But in the real world, there are cases where we want to combine the pros of a log axis with zero or negative values, so in the next Highcharts version we have provided hooks for adding custom conversion functions.