Summary  
This chapter explains how to add and customize trendlines on charts—choosing trendline types (linear, polynomial, moving average), setting forecast ranges, and styling line color, width, and order.

General domain of usage  
Data visualization and forecasting

**Trendlines** help identify patterns and highlight overall trends in data.

In this lesson, you will learn how to add a trendline to a chart and customize its color, width, and style to make trends clearer and easier to interpret.

- Add a **Trendline** using the **Chart Design** tab → **Chart Layouts** group → **Add Chart Element** → **Trendline** (e.g., Linear);
- Open **Format Trendline** by right-clicking the trendline to customize its settings;
- Adjust the **Forecast Forward** value to extend the trendline into future periods;
- Change the trendline type to **Polynomial** and set the required **Order**;
- Modify the **line color** and style in the formatting options to make the trendline more visible.

Initial Workbook

import unittest
import json
import os


def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)


class TestUserCode(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        cls._no_data = True
        cls.sheet = {}
        cls.styles = {}
        if os.path.exists("payload.json"):
            try:
                with open("payload.json") as f:
                    raw = f.read().strip()
                if not raw:
                    return
                data = json.loads(raw)
            except (json.JSONDecodeError, ValueError):
                return
        else:
            return
        cls._no_data = False
        cls.sheet_id = next(iter(data["sheets"]))

        resources = data.get("resources", [])
        cp = next((r for r in resources if r.get("name") == "SHEET_CHART_PLUGIN"), {})
        raw = cp.get("data", "{}")
        chart_data = json.loads(raw) if raw else {}
        cls.charts = chart_data.get(cls.sheet_id, [])

    def _chart_title(self, chart):
        return chart.get("style", {}).get("titles", {}).get("title", {}).get("content", "")

    # ------------------------------------------------------------------ #
    # Step 1 — Linear trendline added to the Income vs Expenses chart
    # ------------------------------------------------------------------ #

    def test_linear_trendline_added(self):
        if self._no_data:
            _dynamic_test(self, True, "Linear trendline added to the Income vs Expenses chart.", ""); return
        if not self.charts:
            _dynamic_test(self, False,
                "Linear trendline added to the Income vs Expenses chart.",
                "No chart found. Make sure you are working on the correct sheet.")
            return

        # Find the Income vs Expenses chart by title, fallback to any multi-series bar chart
        target = [c for c in self.charts if self._chart_title(c) == "Income vs Expenses"]
        if not target:
            target = [c for c in self.charts
                      if set([1, 2, 3]).issubset(set(c.get("context", {}).get("seriesIndexes", [])))]

        if not target:
            _dynamic_test(self, False,
                "Linear trendline added to the Income vs Expenses chart.",
                "Income vs Expenses chart not found. Make sure the chart is still on the sheet.")
            return

        all_trendlines = []
        for c in target:
            all_trendlines.extend(c.get("style", {}).get("trendlines", []))

        has_linear = any(t.get("type") == "linear" for t in all_trendlines)
        _dynamic_test(self, has_linear,
            "Linear trendline added to the Income vs Expenses chart.",
            f"No linear trendline found on the Income vs Expenses chart "
            f"(trendlines present: {all_trendlines}). "
            f"Click the Income vs Expenses chart → top-right button → Setup tab → "
            f"Trendline section → Add Trendline → select Income → confirm.")


if __name__ == "__main__":
    unittest.main()


test_main.py

Dive into the world of Excel with this comprehensive video course, designed to take you from basic navigation to mastering functions and basic data analysis. Starting with foundational skills like data entry and cell formatting, you'll quickly progress to using powerful tools. Whether you're organizing large datasets, performing statistical analysis, or creating dynamic charts, this course equips you with the practical skills needed in today’s data-driven environment.

Kickstart your journey with Excel by understanding its interface, basic functions, and essential data entry techniques. Learn how to navigate toolbars, worksheets, and master the fundamental elements to set a solid foundation.

Dive deeper into Excel by exploring advanced data entry and formatting options. Sharpen your skills in text and number formatting, discover how to effectively use styles, and enhance spreadsheet readability and functionality.

Unleash the power of Excel with an introduction to basic formulas and functions. Gain confidence in creating calculations, using cell references, and employing key functions like SUM, AVERAGE, MIN, and MAX to automate and simplify your data analysis tasks.

In this section, you will master advanced Excel formulas and functions, enhancing your ability to manage complex calculations. Learn to display and audit formulas, utilize nested and logical statements, and apply conditions effectively to streamline your data analysis tasks.

Explore comprehensive techniques for sorting, filtering, and managing data efficiently. This section covers everything from basic data organization to advanced filtering and table management, ensuring you can handle and analyze data effectively.

Dive into the art of data visualization using Excel's powerful charting tools. You'll learn to create various types of charts, customize them to enhance visual appeal, and manipulate data within charts to present clear and compelling visual stories.

Gain proficiency in managing worksheets and workbooks within Excel. This section teaches you how to effectively organize and manipulate workbook elements, utilize formatting tools, and secure your work with passwords, providing a thorough grounding in workbook operations.

Adding Trendlines to Excel Charts

Key Points