How to Fix Your Trendline Not Appearing in ggplot2 Scatterplots

Learn how to ensure your trendline appears in scatterplots created with ggplot2 in R by following these simple steps.
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This video is based on the question https://stackoverflow.com/q/70107287/ asked by the user 'kaix' ( https://stackoverflow.com/u/14568947/ ) and on the answer https://stackoverflow.com/a/70107508/ provided by the user 'rg255' ( https://stackoverflow.com/u/1040035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why Is My Trendline Not Appearing in My Plot?

If you're diving into data visualization with R's ggplot2, you may encounter a frustrating issue: your trendline not showing up in your scatterplot. You've crafted your plot beautifully, yet the trendline seems to be mysteriously absent. Don't worry; this is a common issue and can be fixed with just a tweak or two in your code. Let’s explore why this happens and how to resolve it effectively.

Understanding the Problem

In ggplot2, a trendline usually serves to illustrate the underlying relationship between the variables in your data. In this case, you are trying to show the relationship between the number of new COVID-19 cases and the percentage of people fully vaccinated across various countries. However, when you add a trendline using geom_smooth() without the proper aesthetics, it may not appear as expected.

Here’s the Problematic Code

You might have tried the following line to create your scatterplot:

[[See Video to Reveal this Text or Code Snippet]]

Although this line allows for a colorful scatterplot showing points for different countries, the geom_smooth() for the trendline is not functioning as intended. This is because it’s trying to fit a separate trendline for each color (or country) but no line is displayed since the aesthetics are not set correctly.

Solution: Adjusting Your Code

To fix the issue and ensure your trendline appears, follow the steps laid out below. The solution involves moving the color aesthetic to the geom_point() layer instead of keeping it in the main ggplot call.

Corrected Code

Here's how your code should look after making the necessary adjustments:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes

Move the color Aesthetic: By moving aes(color=Country) into the geom_point() function, you're telling ggplot to differentiate the colors for the points (individual countries), while still allowing geom_smooth() to calculate a single trendline for the overall relationship.

Keep the Trendline in Focus: With this adjustment, geom_smooth() will now create one trendline based on the whole dataset without breaking it down by each specific group.

Conclusion

More often than not, solving plotting issues in R relates to how aesthetics are set up in your ggplot2 functions. By ensuring your trendline is correctly configured, you can effectively visualize important trends in your data. Remember, tweaking aesthetics can hugely impact your plots’ clarity and readability.

Now, fire up your R console, implement this fix and watch your trendline appear as intended on your scatterplot! Happy plotting!