Hope you are talking about the these availability summary report apis.
Availability, which is a percentage value, is calculated for hourly intervals. If we return availability data per minute, for a monitor having the minimum one minute polling interval, the returned value of availability for each minute will be either 100% or 0% except for the minute when the status change has occurred. This becomes more of a heat map than availability percentage chart.
If the heat map is your requirement, you can get the duration for outages and consider monitor is available for the rest of the time. The same api contains "outage_details" node which will give your the exact duration of each outage occurred during the given period.
Let me know if your requirement is different than this.