Question

Further to: Choosing an attractive linear scale for a graph's Y Axis

And what to do when some of the points
are negative?

I believe this part of the question was not answered but it seems I can’t comment or extend that question so I’ve created a new one

Values -100, 0, 100 with 5 ticks:

1. lower bound = -100
2. upper bound = 100
3. range = 100–100 = 200
4. tick range = 40
   1. Divide by 10^2 for 0.4, translates to 0.4, which gives (multiplied by 10^2) 40.
5. new lower bound = 40 * round(-100/40) = -80
6. new upper bound = 40 * round(1+100/40) = 120

or

5. new lower bound = 40 * floor(-100/40) = -120
6. new upper bound = 40 * floor(1+100/40) = 120

Now the range has been increased to 240 (an extra tick!), with 5 ticks at 40 each.
it will take 6 steps to fill the new range!

Solution?

Answer 1

I use the following code. It produces nicely-spaced steps for human viewers and caters for ranges that pass through zero.

public static class AxisUtil
{
    public static float CalculateStepSize(float range, float targetSteps)
    {
        // calculate an initial guess at step size
        float tempStep = range/targetSteps;

        // get the magnitude of the step size
        float mag = (float)Math.Floor(Math.Log10(tempStep));
        float magPow = (float)Math.Pow(10, mag);

        // calculate most significant digit of the new step size
        float magMsd = (int)(tempStep/magPow + 0.5);

        // promote the MSD to either 1, 2, or 5
        if (magMsd > 5.0)
            magMsd = 10.0f;
        else if (magMsd > 2.0)
            magMsd = 5.0f;
        else if (magMsd > 1.0)
            magMsd = 2.0f;

        return magMsd*magPow;
    }
}