You can use funnels to plot conversion rate over time. To do this, build a ratio metric to determine the conversion rate, using the automatically generated <funnel name>.terminal_state funnel metrics :
This example determines the conversion rate of people from step 1 to step 2 in a funnel named "Demo Funnel," using the auto-generated funnel metric
Then you can explore using this metric and group by user attributes. For example, you can see whether Android users converted more quickly than iPhone users.