Openpyxl auto-assigns colors to charts' categories?

I'm writing a function that draw line charts based on data in a worksheet.

In this function, the flag "secondary" checks if the chart will be plotted with a secondary axis, following the example here.

Everything works fine if there are two or more series plotted on the same chart. But if there's just one series, then somehow each time segment is assigned a different color. Why does openpyxl do this and what's the fix?

import openpyxl
from openpyxl import Workbook, load_workbook
from openpyxl.styles import Alignment, NamedStyle
from openpyxl.utils.dataframe import dataframe_to_rows
from openpyxl.utils import get_column_letter
from openpyxl.chart import LineChart, Reference
from openpyxl.chart.axis import ChartLines, TextAxis
from openpyxl.drawing.line import LineProperties
from openpyxl.drawing.text import ParagraphProperties, CharacterProperties
from openpyxl.chart.layout import Layout, ManualLayout
from openpyxl.chart.shapes import GraphicalProperties



def produce_line_chart(ws_data, charts_sheet, series_list, secondary=False):
    """
    Creates a customized line chart based on data from a worksheet and prepares it for placement on a charts sheet.

    Args:
        ws_data (Worksheet): The worksheet containing the data to be charted.
        charts_sheet (Worksheet): The worksheet where the chart will be placed.
        series_list (list): A list of strings where the first item is the chart title and subsequent items are the series names to be charted.
        secondary (bool): If True, disables major gridlines on both axes. Defaults to False.

    Returns:
        tuple: A tuple containing the created chart object and the suggested position for placing the chart on the charts sheet.
    """

    # Create a line chart
    chart = LineChart()

    # Set the title of the chart to the first item in series_list
    chart.title = series_list[0]

    # Increase the chart title's font size to 20pt
    chart.title.tx.rich.paragraphs[0].pPr = ParagraphProperties(
        defRPr=CharacterProperties(sz=2000)  # Font size 20pt
    )

    # Configure the appearance of the axes
    chart.x_axis = TextAxis(tickMarkSkip=4)
    chart.x_axis.spPr = GraphicalProperties(ln=LineProperties(solidFill="D9D9D9"))
    chart.y_axis.spPr = GraphicalProperties(ln=LineProperties(solidFill="D9D9D9"))

    # Add or remove major gridlines based on the 'secondary' flag
    if secondary:
        chart.x_axis.majorGridlines = None
        chart.y_axis.majorGridlines = None
    else:
        chart.x_axis.majorGridlines = ChartLines()
        chart.x_axis.majorGridlines.spPr = GraphicalProperties(
            ln=LineProperties(solidFill="D9D9D9")
        )
        chart.y_axis.majorGridlines = ChartLines()
        chart.y_axis.majorGridlines.spPr = GraphicalProperties(
            ln=LineProperties(solidFill="D9D9D9")
        )

    # Set tick marks on the axes
    chart.x_axis.majorTickMark = "out"
    chart.y_axis.majorTickMark = "cross"

    # Ensure axes are displayed
    chart.x_axis.delete = False
    chart.y_axis.delete = False

    # Set the legend position at the bottom of the chart
    chart.legend.position = "b"

    # Adjust the layout to ensure enough space for the legend
    chart.layout = Layout(
        manualLayout=ManualLayout(
            x=0,  # x position of the plot area
            y=0,  # y position of the plot area
            h=0.85,  # height of the plot area
            w=0.9,  # width of the plot area
        )
    )

    # Scale the chart by adjusting its height and width
    chart.height = chart.height * 1.71
    chart.width = chart.width * 2.03

    # Get the first row of the worksheet data to identify the series columns
    first_row = next(ws_data.iter_rows(min_row=1, max_row=1, values_only=True), [])

    # Loop through each series name in series_list (excluding the title)
    num_rows = ws_data.max_row
    for series in series_list[1:]:
        for idx, cell_value in enumerate(first_row, start=1):
            if cell_value == series:
                col = idx
                break
        # Create a reference to the data for the series and add it to the chart
        series_to_chart = Reference(
            ws_data, min_col=col, min_row=1, max_col=col, max_row=num_rows
        )
        chart.add_data(series_to_chart, titles_from_data=True)
        # Ensure the series line is not smoothed
        chart.series[-1].smooth = False

    # Set the categories (x-axis labels) based on the first column of data (excluding the header)
    categories = Reference(ws_data, min_col=1, min_row=2, max_row=ws_data.max_row)
    chart.set_categories(categories)

    # Determine the next available position for the chart on the charts sheet
    chart_row = 1  # Start in the first row
    for existing_chart in charts_sheet._charts:
        existing_chart_row = int("".join(filter(str.isdigit, existing_chart.anchor)))
        # Convert the chart height from cm to Excel cell height and add some space
        chart_row = max(chart_row, existing_chart_row + int(chart.height * 1.86) + 4)

    # Determine the position to add the chart on the charts sheet
    chart_position = f"A{chart_row}"

    return chart, chart_position

The problem depends on the Version of Microsoft Excel which shows the chart.

There is a setting for varying the colors of same-series data markers in a chart. See: Vary the colors of same-series data markers in a chart. That was false per default up to Excel 2007. Then Microsoft decided to make that setting true per default. And from Microsoft 365 on it is false per default again. So you need explicitly set that false if you do not want it in any Microsoft Excel version.

For openpyxl see LineChart.varyColors. The default is not to set that. Then the Excel version default get used.

Minimized use case:

from datetime import date

from openpyxl import Workbook 
from openpyxl.chart import (
    LineChart,
    Reference,
)

wb = Workbook()
ws = wb.active

rows = [
    ['Date', 'Ser 1'],
    [date(2015,9, 1), 40],
    [date(2015,9, 2), 40],
    [date(2015,9, 3), 50],
    [date(2015,9, 4), 30],
    [date(2015,9, 5), 25],
    [date(2015,9, 6), 20],
]

for row in rows:
    ws.append(row)

chart = LineChart()
chart.title = "Line Chart"
chart.y_axis.title = 'Value'
chart.x_axis.title = 'Date'

data = Reference(ws, min_col=2, min_row=1, max_col=2, max_row=7)
chart.add_data(data, titles_from_data=True)
dates = Reference(ws, min_col=1, min_row=2, max_row=7)
chart.set_categories(dates)

#chart.varyColors = True
chart.varyColors = False

ws.add_chart(chart, "A10")

wb.save("line.xlsx")

There chart.varyColors = True will always show each data point in another color. While chart.varyColors = False will never do this.