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

def transactie_analyse (filename):
    wb = xl.load_workbook('filename')  
    sheet = wb ['Blad']
    
    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,'cel')
    
    wb.save(filename)