import openpyxl as xl

from openpyxl.charts import BarChart, Reference

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

# cell = sheet["A1"]
# print(cell.value)

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

Reference(sheet,
          min_row=2,
          max_row=sheet.max_row,
          min_col=4,
          max_col=4)

chart = BarChart()
chart.add_data(values)

wb.save('transactions3.xlsx')