What you want is the function 'timedsequence'.
Something like this that you apply to X of your source:
xtopercent(timedsequence(-1,0,grid.onactivated,20,0,10,1,25,2))
Same formula with comments to help you understand and tweak values:
xtopercent(timedsequence(
-1, // how many times it repeats, -1 for infinite
0, // crossfade duration
grid.onactivated, // reset when grid is activated
20, // duration of first step
0, // which cell first step
10, // duration of second step
1, // which cell second step
25, // duration of third step
2 // which cell third step
))
TIP: You can click the arrow in the formula box to make it bigger