SSRS - Using a report Text Field in a Chart's Calculation (ReportItems)

Access the textbox value via ReportItems collection by:

ReportItems!textbox.Value


Quick example, this was taken from the MarketCapAnalysis.rdl file in the standard reports:

=Format(cdbl(Fields!Field_Value.Value)/cdbl(ReportItems!textbox3.Value), "#,##0.00%")


Note: remember to convert before using the val in a calculation!​