Although it might possibly be true as some of the answers and comments state that good graphics can be produced in Excel, there is one very big downside to using Excel for scientific graphics. The major downside is that it is very easy to make mistakes in an Excel spreadsheet and very difficult to see that they have been made. Auditing a spreadsheet is exceptionally challenging - you basically have to try to reproduce the results by other means. There is a way round that problem but it involves adhering to extremely strict protocols as to the construction of the spreadsheet. Since most people produce complicated spreadsheets by starting simple and adding complications, the result will generally be unauditable (other than by reproduction).
ggplot is free to use and very easy to learn.