标题：为何不能生成图表

公式为:

dim a as winform.chart
dim b as winform.chartseries
dim c as list (of string)
c = datatables("订单").getuniquevalues("","产品")
a = forms("窗口2").controls("chart1")
a.serieslist.clear()
b = a.serieslist.add()
b.length = c.count
for  i as integer = 0 to c.count-1
 b.x(i) = i
 b.y(i) = datatables("订单").compute("sum(金额)","[产品] = '"& c(i) &"' ")
 a.axisx.setvaluelabel(i,c(i))
next
 a.AxisX.AnnoWithLabels = True

哪里错了????