try to handling the bug of axis stroke style

Author: xieguigang

test/demo/plotxy.R

@@ -0,0 +1,6 @@
+let x = (-10):10 step 0.1;
+let f(x) = x^3;
+
+svg(file = file.path(@dir,"plotxy.svg")) {
+    plot(x,f(x));
+}