Vyxal, 1316 bytes
ƛ¦Þǔ;∑¦Ḣ:G/*⌈*⌈×*⁋
Straightforward port of my Jelly answer, go see that explanation for a better idea of how it works. Jelly has some nicer builtins.
ƛ ; # Map each task to...
¦ # Cumulative sums (instants where a step will complete)
Þǔ # Untruth (a boolean list with 1s at those indices)
∑ # Reduce the whole thing by addition
¦Ḣ # Get cumulative sums and remove the leading zero
:G/ # Divide by the maximum
* # Multiply by the input
⌈ # Get the ceiling
×*⁋ # Make a bar graph