Fit the Line

Drag the line. Beat the best fit.

Select mode

How to play

The idea

You see a scatter plot. Place a straight line through the dots where you think the best fit is. Then the game shows the true least-squares regression line and scores how close you were.

Playing solo

Look at the dots. The line should run through the middle of the cloud, with about as many dots above it as below it.
Drag the two handles to move your line. With the keyboard, focus a handle and use the arrow keys (hold Shift for bigger steps).
Press Check line. The pink line is the true least-squares line, and the shaded area is the gap between it and yours.
Your score for the round is 0 to 100. Play 10 rounds for a total out of 1000. Your best average for each difficulty is saved on this device.

Difficulty

Easy has tightly grouped dots.
Medium is more spread out.
Hard is the loosest and adds two stray points that pull the true line toward them.

After you check

You see both equations, the sum of squared residuals (SSE) for each line, and the correlation r. The menu under the plot can draw the residuals, which are the vertical distances from each dot to a line, for your line or the best-fit line.

How scoring works

The game measures the average vertical gap between your line and the least-squares line across the whole graph.

score = 100 x (1 - gap / 2), never below 0

A perfect match scores 100. An average gap of 2 units or more scores 0.

Playing in a class

Your teacher launches a class and puts a 4-letter code on the board.
On the title screen, press Join a class, type the code and your first name or a nickname, and press Join.
Wait for the teacher to start. Everyone plays the same dots in each round.
Drag your line, then press Lock in. You cannot change it after that.
When the teacher reveals the answer, you see your score and the class scoreboard. Closing the page by accident? Use Rejoin on the title screen.

Teachers

Press Launch a class game, choose a difficulty and number of rounds, and press Create.
Show the code on the board. Student names appear as they join, and you can remove a name with the x.
Press Start game. When your students have locked in, press Reveal, talk through the result, then press Next round.
At the end you get a podium and a CSV download of the scores. End class deletes the class and everyone's names.

About

About Fit the Line

Fit the Line is a free classroom game for learning to judge a line of best fit by eye. It was made by Brock Hammill, a high school science teacher in Corvallis, Montana.

It is an independent project. It is not an official product of any school or school district.

Questions, problems, or requests to delete something: [email protected].

Privacy policy

Effective October 11, 2026.

The short version: there are no accounts, no ads and no tracking. The game only keeps a name you type and your lines, and only for class games.

What the game collects

Solo play: nothing is sent anywhere. Your best scores are saved in your own browser.
Joining a class: the class code, the name you type (up to 16 characters), the line you lock in each round, and the time you joined. The game also creates a random anonymous ID so it can tell players apart. It is not linked to your name, email or any account.
Launching a class: the class settings (difficulty, number of rounds) and a random anonymous ID for the teacher's browser.

The game does not ask for email addresses, passwords, ages, photos, locations or contact details. Please use a first name or nickname, never a full name or anything private. Your name is shown to your teacher and to the other people in your class on the scoreboard, and your teacher can download scores as a spreadsheet.

What is stored in your browser

The game saves a few things on your device: best scores, the class you are currently in (so Rejoin works), a teacher's most recent class code, and the anonymous sign-in. You can clear these any time by clearing this site's data in your browser settings.

Services that help run the game

Google Firebase stores class data and the anonymous sign-in. See Google's privacy policy.
Google Fonts supplies the game's fonts, so your browser contacts Google when the page loads.
GitHub Pages hosts the site. See GitHub's privacy statement.
Cloudflare handles the fittheline.com address and passes traffic to the site. See Cloudflare's privacy policy.

These providers can see technical information such as your IP address when your browser connects, as part of how the internet works. I do not sell your information, share it for advertising, or use analytics or advertising cookies.

How long class data is kept

When a teacher presses End class, the class and every player's name and lines are deleted. A class that is not ended is also set to expire about a day after it was created and is then removed automatically, which can take a little longer. Solo data never leaves your device.

Students and schools

This game is meant to be used in classrooms, including by students under 18. It collects only the name a student types. Teachers are responsible for following their school's and district's rules about students using online tools. If a student's personal information ended up in the game by mistake, email me and I will delete it.

Security

Connections to the site are encrypted. Only the browser that created a class can change or delete it, and students can only add their own line for the current round. No online service is perfectly secure, which is another reason to keep names simple.

Changes and contact

If this policy changes, the new version will be posted here with a new date. Contact: [email protected].

Terms of service

Effective October 11, 2026.

By using Fit the Line you agree to these terms. If you are a student, your teacher decides whether and how you use the game.

Using the game

The game is free to use for learning and teaching.
Use a respectful name. Do not use offensive names, other people's names, or personal information as your name.
Do not try to break, overload, scrape or get around the game, read other people's data, or interfere with other people's classes.
Teachers can remove players from their class. I may remove names or classes that break these rules.

No guarantees

The game is provided as is, without warranties of any kind. It is a practice tool, not a formal assessment, so please do not use it for grades. It may have mistakes, may be unavailable at times, and may change or stop at any time. Class data can be lost, for example if the service has a problem.

Ownership

Limits on liability

To the fullest extent the law allows, I am not responsible for any loss or damage that comes from using or not being able to use the game.

Changes, law and contact

I may update these terms and will post the new version here with a new date. Using the game after a change means you accept it. These terms are governed by the laws of the State of Montana. Contact: [email protected].

Fit the Line

How to play

The idea

Playing solo

Difficulty

After you check

How scoring works

Playing in a class

Teachers

About

About Fit the Line

Privacy policy

What the game collects

What is stored in your browser

Services that help run the game

How long class data is kept

Students and schools

Security

Changes and contact

Terms of service

Using the game

No guarantees

Ownership

Limits on liability

Changes, law and contact

Join a class

Launch a class game

Class

Players

Final standings

Fit the Line