SPC Referance lines are not dynamic

[helen_pip]

Hello

I have a graph in qlikview which shows a count over the last 2 years

I have polotted an upper and lower control limit in the reference lines, to form an SPC chart

Upper Limit 

Avg

(Aggr(COUNT(LOCAL_PATIENT_IDENTIFIER), OutPatients_Week)) + (Stdev(Aggr(COUNT(LOCAL_PATIENT_IDENTIFIER), OutPatients_Week)*3))

Lower Limit

Avg

(Aggr(COUNT(LOCAL_PATIENT_IDENTIFIER), OutPatients_Week)) - (Stdev(Aggr(COUNT(LOCAL_PATIENT_IDENTIFIER), OutPatients_Week)*3))

The reference lines show when no selections are made from the list boxes, and I have the static max set to 5500

However the lines are not dynamic and when selections are made from list boxes, the lines dissaper off the graph.  Forced 0 does not work either.   I do not want to set a static max, as the data I am counting will not show peaks and troughs in the graph

I ahve tried copying the lines as an expression, but they dissapear off the graph altogether

My question is, how can I show the Upper and Lower control limits as dynamic when the user makes selections?

Any ideas?

Thanks

Helen