logarithm graph- how?? NEED HELP

how to make a logarithm base a (y=log a x) graph using VB?
when we insert a value, the graph will change base on a value.

i have search it's function...but i still dont understand and dont know how to built the function.

i have search about the logarithm..
http://en.wikipedia.org/wiki/Logarithm

and the graph is :
http://en.wikipedia.org/wiki/Image:Logarithms.png

so, can we just insert the value a and make the logarithm graph?
if can, how to make the function?