test.py

import openpyxl as xl
from openpyxl.chart import BarChart, Reference
def process_workbook(filename):
    wb = xl.load_workbook(filename)
    sheet = wb["Sheet1"]
    new_head = sheet.cell(1,4)
    new_head.value = "new_price"
    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=4,min_col=4,max_col=4)

    chart = BarChart()
    chart.add_data(values)
    sheet.add_chart(chart,"e2")

    wb.save(filename)
process_workbook("transactions.xlsx")