Excel 2007 : Please tell me why I get a "Run-time error '9' Subscript out of range"

Hello again!

In the code below I am attempting to add a new series to a chart, after the user defines the Y-axis value to use in drawing the line. They get three lines to define (High Alarm, Set Point and Low Alarm) and I have only added the new series code to the first possibility, where they entered a value in cell $D$4. Note: The graph is on the sheet "Cond Graph" as is the cells where they enter the y-axis values, but the data used by the graph is on the sheet "AIO-Cond".

When I enter a new value into "Cond Graph" cell $D$4, the code below halts with an error:
"Run-time error '9':
Subscript out of range"

And the line in the code below that is highlighted by the Debug is (not quite halfway down):
"With Worksheets("Cond Graph")"

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As usual, I have no idea what that error means, or more correctly why I am getting that error. Can any of you nice folks help?

- Thomas