Pie chart not showing

Dash Python
1

I am very new to dash and have been toiling around this problem for while, could anyone help?

ive got a daterangepicker which then finds values in a df, does some caluclations and finds pecentages in integers.

The calculations work find because if i ask the dash to return the percentage split (e.g 60,40) it shows me the integers perfectly, its only when i return the data and layout does an error message come up as

navy_trouser_culm=int(((len(ref_df[ref_df['product_title'].str.contains('Navy Blue')]))/(len(ref_df)))*100)
ZeroDivisionError: division by zero

I have no idea how this could be possible as ive checked the numbers and there is no 0, and as ive said before when i return navy_trousers_culm to be printed in the dashboard it says gives a number.

Below is my code:

app = dash.Dash()

app.layout = html.Div([

    dcc.DatePickerRange(
        id='my-date-picker-range',
        min_date_allowed=dt(2019, 4, 1),
        max_date_allowed=dt(2019, 6, 30),
        #initial_visible_month=dt(2019, 4, 1),
        end_date=dt(2019, 4, 2),
        start_date=dt(2019, 4, 1)
    ),
    html.Div(id='output-container-date-picker-range')
])


@app.callback(
    dash.dependencies.Output('output-container-date-picker-range', 'children'),
    [dash.dependencies.Input('my-date-picker-range', 'start_date'),
     dash.dependencies.Input('my-date-picker-range', 'end_date')]) 

def update_output(start_date, end_date):
    
    start_day=str(start_date.split('-')[2]).split('T')[0]
    start_month=str(start_date.split('-')[1])
    start_year=str(start_date.split('-')[0])
    
    end_day=str(end_date.split('-')[2]).split('T')[0]
    end_month=str(end_date.split('-')[1])
    end_year=str(end_date.split('-')[0])
    
    start_date_ref=start_day+'/'+start_month+'/'+start_year
    end_date_ref=end_day+'/'+end_month+'/'+end_year
     
    #finds earliest and latest dates for start_date_ref and end_date_ref
    #if the dates arent available in the data
    
    if start_date_ref not in days:
        start_date_ref=find_earlier_start_date(start_date_ref)
     
    if end_date_ref not in days:
        end_date_ref=find_later_end_date(end_date_ref)
        
    start_index=list(df[df['day']==end_date_ref].index.values)[0]
    end_index=list(df[df['day']==start_date_ref].index.values)[-1]
    
    #as df is ordered earliest at the top the end date is the start index
    
    ref_df=df[start_index:end_index+1]
    #return(str(len(ref_df)))
    
    navy_trouser_culm=int(((len(ref_df[ref_df['product_title'].str.contains('Navy Blue')]))/(len(ref_df)))*100)
    sand_trouser_culm=int(((len(ref_df[ref_df['product_title'].str.contains('Sand')]))/(len(ref_df)))*100)
    return{
            'data': [go.Pie(labels=['Navy','Sand'], values=[50,50],
                             marker={'colors': ['#EF963B','C93277']},textinfo='label')
                    ],
               
            'layout': go.Layout(
                title='distribution',
                margin={'l': 100, 'r': 10},
                legend={'x': 0, 'y': 1.5},
             
            )
        }
              
if __name__ == '__main__':
    app.run_server()