Algorithm for “nice” grid line intervals on a graph

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I’ve done this with kind of a brute force method. First, figure out the maximum number of tick marks you can fit into the space. Divide the total range of values by the number of ticks; this is the minimum spacing of the tick. Now calculate the floor of the logarithm base 10 to get the magnitude of the tick, and divide by this value. You should end up with something in the range of 1 to 10. Simply choose the round number greater than or equal to the value and multiply it by the logarithm calculated earlier. This is your final tick spacing.

Example in Python:

import math

def BestTick(largest, mostticks):
    minimum = largest / mostticks
    magnitude = 10 ** math.floor(math.log(minimum, 10))
    residual = minimum / magnitude
    if residual > 5:
        tick = 10 * magnitude
    elif residual > 2:
        tick = 5 * magnitude
    elif residual > 1:
        tick = 2 * magnitude
    else:
        tick = magnitude
    return tick

Edit: you are free to alter the selection of “nice” intervals. One commenter appears to be dissatisfied with the selections provided, because the actual number of ticks can be up to 2.5 times less than the maximum. Here’s a slight modification that defines a table for the nice intervals. In the example, I’ve expanded the selections so that the number of ticks won’t be less than 3/5 of the maximum.

import bisect

def BestTick2(largest, mostticks):
    minimum = largest / mostticks
    magnitude = 10 ** math.floor(math.log(minimum, 10))
    residual = minimum / magnitude
    # this table must begin with 1 and end with 10
    table = [1, 1.5, 2, 3, 5, 7, 10]
    tick = table[bisect.bisect_right(table, residual)] if residual < 10 else 10
    return tick * magnitude

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