Change timeline curve type to step-after

[mootari]

Describe the feature

The timeline currently interpolates between data points, which misrepresents the data because sizes don't increase gradually between releases.

We should switch the curve type to step-after, so that increments only happen at the time of release.

Context:

• Suggestion and discussion: Discord
• Example: Discord

Additional information

• Would you be willing to help implement this feature?

Final checks

• Read the contribution guide.
• Check existing issues.