By default in locked file end user has ability modify Charts and Objects. For example he can change text on button or formatting on a Chart. To prevent this you should use usual Excel's protection and protect sheet with a password with option "Edit objects" unchecked.
LockXLS protects file from password cracking applications, does not allow to unprotect this sheet and keeps formulas unavailable for customer, while input cells can be modified and changed.
There is a sample project, which can be downloaded from our website Charts.zip.
It shows spreadsheet with chart, where cells can be modified but formulas and chart - does not. We've done the following with this file: