[rrd-users] showing percentage of service availability per certain

Hello,
I'm trying to figure out how to configure rrd graph tool to get me the
percentage of server availability.
I'm monitoring servers which returns round-trip time to rrdtool.
Sometimes(when the server doesn't respond) I don't get any answer within
given period of time so in rrd DB is NaN.
I would like to calculate for certain period of time e.g. a week how
many times the server was not accessible.
Something like the successful number of tests divided by total amount of
tests for the period.

Could you give me an example of this approach as rrd script?

Thank you,
Lada

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