initBorder method
计算边界
Implementation
void initBorder(Size size) {
startX = yNum! > 0 ? basePadding * 2.5 : basePadding * 2; //预留出y轴刻度值所占的空间
endX = size.width - basePadding * 2;
startY = size.height - (isShowXyRuler ? basePadding * 3 : basePadding * 2);
endY = basePadding * 2;
_fixedHeight = startY - endY;
_fixedWidth = endX - startX;
maxMin = calculateMaxMin(chartBeans);
}