Altair Finance Plot

finance.py

# This is a variable forecast for modelling different growth-rates.
# Initial growth rate is set to be the current growth rate

# This cannot be 0, as growth rate wouldn't work
start_revenue = INITIAL_REVENUE if INITIAL_REVENUE > 100 else 100
scale = (-CURRENT_CASH_GBP, CURRENT_CASH_GBP * 2)

monthly_growth_percent = WEEKLY_GROWTH_PERCENT * 4
slider = alt.binding_range(min=0, max=100, step=0.1, name='Monthly growth rate (%)')
selector = alt.selection_single(fields=['month_growth'], bind=slider, init={'month_growth': monthly_growth_percent})

base = alt.Chart(df[['date', 'cash_0g']]).transform_window(
    # Use count() to find out the current month we're on (i.e. the index), as we use that as the power for our growth rate
    index='count()'
).transform_calculate(
    # Transform a % growth rate (i.e. 7) into sometime useful (i.e. 1.07).
    # Raise that to our current month (i.e. for month 2, 1.07 ** 2 = ~1.145), and multiply that by our revenue amount (~1.145 * 500)
    forecast_revenue = ((1+(selector.month_growth / 100)) ** alt.datum.index) * start_revenue
).transform_window(
    # Take what is in practice the cumulative product of revenue (i.e. total revenue earned so far)
    forecast_total_revenue = 'sum(forecast_revenue)', frame=[None, 0]
).transform_calculate(
    # Add cumprod to current cash to find cash standing (i.e. if we have £100k cash, and 10 months of £10k revenue, we have £200k now)
    adj_cash = alt.datum.cash_0g + alt.datum.forecast_total_revenue
).encode(x='yearmonth(date):T')

cash = base.encode(y=alt.Y('adj_cash:Q', scale=alt.Scale(domain=scale))).mark_area(opacity=0.75)
revenue = base.encode(y='forecast_total_revenue:Q').mark_line(color='#f26522', size=5) 
revenue_growth = base.encode(y='forecast_revenue:Q').mark_line(color='purple', size=5)

charts = cash + revenue + revenue_growth

chart_final = charts.add_selection(selector).interactive().properties(width=850)