调用示例：
(1) plotsmithchart(25,50)
Draws a smithchart,calculates and plots the reflection coefficient for a characteristic impedence of 50 ohms and a load impedence of 25 ohms.
(2) plotsmithchart(50+50j,75)
Same as (1) with a load impedence of 50+50j ohms.
(3) plotsmithchart
Plots a blank Smith Chart.