Not sure about over and under Null as Null doesn't exist, but over and under 0 can be found summarised with these fields:
if(Savings>0,Savings) AS OverSavings,
if(Savings<0,Savings) AS UnderSavings,
if(Savings>0,1) AS OverCount,
if(Savings<0,1) AS UnderCount,
FROM. ... TableSource;
Should be able to do Sum(OverCount) and Sum(UnderCount) in a chart.