グラフの最適な目盛りの求め方。　参考にさせていただいたサイト https://stackoverflow.com/questions/361681/algorithm-for-nice-grid-line-intervals-on-a-graph　https://oshiete.goo.ne.jp/qa/2322056.html

optimizedGraphTick.py

import math
import sys


def computeTickWidth(range, mostDivision):
    minInterval = range / mostDivision
    magnitude = pow(10, math.floor(math.log10(minInterval)))
    residual = minInterval / magnitude

    if residual > 5:
        tickWidth = 5 * magnitude
    elif residual > 2.5:
        tickWidth = 2.5 * magnitude
    elif residual > 2:
        tickWidth = 2 * magnitude
    else:
        tickWidth = magnitude
    return tickWidth


def computeAxisRange(min, max, tickWidth):
    axisMax = 0.0
    axisMin = 0.0
    if min < 0 < max:
        while abs(axisMax - max) > sys.float_info.epsilon and axisMax < max:
            axisMax += tickWidth

        while abs(axisMin - min) > sys.float_info.epsilon and axisMin > min:
            axisMin -= tickWidth
    else:
        axisMin = min
        axisMax = min
        while abs(axisMax - max) > sys.float_info.epsilon and axisMax < max:
            axisMax += tickWidth
    return [axisMin, axisMax]


#------------------------------------------------------------------------
# Example
# You can change MAX, MIN, MOSTDIV values.
MAX = 1.2
MIN = -0.9
RANGE = MAX - MIN
MOSTDIV = 4
tick = computeTickWidth(RANGE, MOSTDIV)
minAxisValue, maxAxisValue = computeAxisRange(MIN, MAX, tick)
print("AxisMax: ", maxAxisValue)
print("AxisMin: ", minAxisValue)
print("TickWidth: ", tick)
#------------------------------------------------------------------------
# AxisMax:  1.5
# AxisMin:  -1.0
# TickWidth:  0.5