Allow for demand taxes

[santiago-afonso]

Hello. Thanks for this incredible library!
Here's a suggestion: tax_graph() only allows for supply taxes. For didactic purposes, it should also allow for demand taxes, to show this:

Thanks!

[R-CoderDotCom]

Hi @osnofas, aright now I don't have enough time for this package (I'm only a single person creating all the packages and content of the R CODER site). I will try to come back with econocharts development in a near future. Thanks for this suggestion and #7