How to enable grid lines in Matplotlib

Grid lines make a Matplotlib chart easier to read when values need to be compared against tick marks rather than only against the plotted line. Keeping them light and behind the data gives line charts, bar charts, and scatter plots visible reference points without changing the plotted values.

Axes.grid() changes the grid on the specific Axes object, so the object-oriented interface keeps the setting attached to the chart being exported. Use axis=“y” when horizontal reference lines are enough, axis=“both” when both directions help, and which to choose major ticks, minor ticks, or both.

Minor grid lines need minor tick positions before they can appear. Set a minor locator such as AutoMinorLocator(), keep the grid below the plotted data with set_axisbelow(True), and use a light color or alpha so the grid supports the data instead of competing with it.

1. Save the plotting script as grid_enable.py.

   grid_enable.py

   from pathlib import Path
    
   import matplotlib.pyplot as plt
   from matplotlib.ticker import AutoMinorLocator, MultipleLocator
    
    
   output = Path("grid-enable.png")
    
   weeks = ["W1", "W2", "W3", "W4", "W5", "W6"]
   incidents = [48, 43, 39, 41, 35, 31]
    
   fig, ax = plt.subplots(figsize=(6.2, 3.6), layout="constrained")
   ax.plot(weeks, incidents, marker="o", linewidth=2.2, color="tab:blue")
   ax.set_title("Open incidents by week")
   ax.set_xlabel("Week")
   ax.set_ylabel("Open incidents")
   ax.set_ylim(25, 55)
   ax.yaxis.set_major_locator(MultipleLocator(10))
   ax.yaxis.set_minor_locator(AutoMinorLocator(2))
   ax.set_axisbelow(True)
   ax.grid(True, axis="y", which="major", color="0.78", linewidth=0.9)
   ax.grid(True, axis="y", which="minor", color="0.90", linestyle=":", linewidth=0.6)
    
   fig.canvas.draw()
    
   major_y_grid = [line for line in ax.get_ygridlines() if line.get_visible()]
   minor_y_grid = [
       tick.gridline for tick in ax.yaxis.get_minor_ticks() if tick.gridline.get_visible()
   ]
    
   fig.savefig(output, dpi=160)
   plt.close(fig)
    
   print(f"major y grid lines: {len(major_y_grid)}")
   print(f"minor y grid lines: {len(minor_y_grid)}")
   print(f"axis below data: {ax.get_axisbelow()}")
   print(f"saved: {output}")
   print(f"bytes: {output.stat().st_size}")

   axis=“y” draws horizontal grid lines only. Use axis=“both” when vertical grid lines make the x positions easier to compare.

2. Run the script from the directory where the PNG should be written.

   $ python grid_enable.py
   major y grid lines: 5
   minor y grid lines: 4
   axis below data: True
   saved: grid-enable.png
   bytes: 36261

   The byte count can change with Matplotlib, font, backend, or DPI differences. A nonzero byte count confirms that the PNG was written.

3. Open grid-enable.png and confirm the grid appears behind the plotted line.

   The plot should show solid horizontal major grid lines at the labeled y ticks and lighter dotted minor grid lines between them.

4. Enable only major grid lines when minor reference lines are not needed.

   ax.grid(True, axis="both", which="major", color="0.82", linewidth=0.8)

   Passing style keyword arguments such as color, linestyle, linewidth, or alpha also turns the grid on.

5. Add minor grid lines only after minor ticks or a minor locator exists.

   ax.yaxis.set_minor_locator(AutoMinorLocator(2))
   ax.grid(True, axis="y", which="minor", color="0.90", linestyle=":", linewidth=0.6)

   Minor ticks are off by default in many plots. Without minor tick locations, which=“minor” has no minor positions to draw.

6. Keep dense plots readable by drawing the grid under the data.

   ax.set_axisbelow(True)

   Use set_axisbelow(“line”) when grid lines should sit above filled patches but below line and marker artists.

7. Remove the sample files when they were created only for testing.

   $ rm grid_enable.py grid-enable.png