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

wb = xl.load_workbook('transactions.xlsx')
sheet = wb['Sheet1']

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

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('transactions2.xlsx')