import openpyxl as xl
from openpyxl.chart import BarChart,Reference

def process_workbook(filename):
    wb=xl.load_workbook(filename)
        sheet=wb['Sheet1']

    for row in range(2,sheet.max_row+1):
        cell=sheet.cell(row,3)
        corrected__price_cell=cell.value*0.9
        sheet.cell(row,4).value=corrected__price_cell

    values=Reference(sheet,min_row=2,max_row=sheet.max_row,min_col=4,max_col=4)
    chart=BarChart()
    chart.add_data(values)
    sheet.add_chart(chart,'e2')
    wb.save(filename)
    
