The 95% line is only available if your graph is generated with PNP4Nagios.
If you use the enterprise edition, then it is not possible to draw such a line.
The function used inside the PNP4Nagios is the following - taken from rrdtool manual
https://oss.oetiker.ch/rrdtool/doc/rrdgraph_rpn.en.html
PERCENT, PERCENTNAN
This should follow a DEF or CDEF vname . The vname is popped, another number is popped which is a certain percentage (0…100). The data set is then sorted and the value returned is chosen such that percentage percent of the values is lower or equal than the result. For PERCENTNAN Unknown values are ignored, but for PERCENT Unknown values are considered lower than any finite number for this purpose so if this operator returns an unknown you have quite a lot of them in your data. Inf inite numbers are lesser, or more, than the finite numbers and are always more than the Unknown numbers. (NaN < -INF < finite values < INF)
Example: VDEF:perc95=mydata,95,PERCENT VDEF:percnan95=mydata,95,PERCENTNAN