[noisy_boy]

To those saying it doesn't look good, why not make the dots like levels?

Say you have only 5 colors: green, blue, orange, purple, red.

    1st year: green
    2nd year: replace the green dot with blue
    3rd year: replace the blue dot with orange
    4th year: replace the orange dot with purple
    5th year: replace the purple dot with red
    6th year: red + add green dot
    7th year: red + replace the green dot with blue

Even if you use a box for 10 years, you will have max two dots. Sure, granularity is only yearly. An alternative refinement is to continue with the current system but collapse all the 1st year dots with a single next year's dot.

[bluGill]

Because you need to do something yearly which is tedious and so a task you are likely to postpone. Does it really matter if the green dot on the box is from this year or 10 years ago? The important signal is the box with the red dot hasn't been touched in 9 years so if you need some space that is a candidate for getting rid of.