creating nice numbers - how to deal with numbers < 0 with log10?

Question

I put together a function to create "nice numbers" for displaying labels. The function makes use of log10 to normalize odd numbers. Unfortunately, log10 doesn't deal with numbers < 0 but returns NAN.
Now, I have an "ugly" number like -2.36, how to get the nearest nice number, like -2.3or -2.0?

If I convert it to 2.36 in order to put it thru the function and multiply it by -1 afterwards, I'd get -2.4 ---> no go, because it is required that nice-number > ugly-number.

Any ideas?

Links to define nice-numbers etc:
http://wiki.tcl.tk/16640
Algorithm for "nice" grid line intervals on a graph
round to nearest nice number

Here's my code:

function niceNum($range, $round) {
    // $exponent: exponent of range
    // $fraction: fractional part of range
    // $niceFraction: nice, rounded fraction  
    if ($range==0) return 0;

    $exponent = floor(log10($range));
    $fraction = $range / pow(10, $exponent);

    if ($round) {
        if ($fraction < 1.5)
            $niceFraction = 1;
        elseif ($fraction < 3)
            $niceFraction = 2;
        elseif ($fraction < 7)
            $niceFraction = 5;
        else
            $niceFraction = 10;
    } else {
        if ($fraction <= 1)
            $niceFraction = 1;
        elseif ($fraction <= 2)
            $niceFraction = 2;
        elseif ($fraction <= 5)
            $niceFraction = 5;
        else
            $niceFraction = 10;
    }
    return $niceFraction * pow(10, $exponent);
}