/// <summary>
/// Adds an income to the plan.
/// </summary>
/// <param name="id">The id of the income to use.</param>
/// <param name="amount">How much the monthly income is.</param>
public void AddIncome(int id, double amount)
{
var income = new Income(id, amount);
mPlan.AddIncome(income);
mPlanRepository.UpdatePlan(mPlan);
}