"用图解法表示" can be translated into English as "represent (or depict, illustrate) graphically" or "use graphic representation to show".
Here are some example sentences:
We can represent the data graphically to make it easier to understand. （我们可以用图解法表示数据，以便更容易理解。）
The relationship between the variables is best illustrated graphically. （变量之间的关系最好用图解法表示。）