Created attachment 37809 [details]
test case for blank cells in range calc

Hi, attached excel file shows different Excel/poi 4.1.2 Slope/Intercept calcs caused by the slope/intercept cell range containing blank cells. Code use to generate Excel file:
public static void main(String[] args) {
		try {
			File initialFile = new File(
					"C:\\Users\\twelch\\eclipse-workspace\\Main\\test_excel\\test_slope_intercept.xlsx");
			InputStream is = new FileInputStream(initialFile);
			XSSFWorkbook workbook = new XSSFWorkbook(is);
			is.close();
			
			// Excel formulas : D2 slope, D3 intercept
			// evaluate and save results in F2, F3
			XSSFCell slopeCell = cellAtAddr(workbook, "Sheet1!D2");
			XSSFCell interceptCell = cellAtAddr(workbook, "Sheet1!D3");
			
			evalCellSaveAtAddr(workbook, slopeCell, "Sheet1!F2");
			evalCellSaveAtAddr(workbook, interceptCell, "Sheet1!F3");
						
			workbook.setForceFormulaRecalculation(true);
			File outFile = new File(
					"C:\\Users\\twelch\\eclipse-workspace\\Main\\test_excel\\test_slope_intercept.xlsx");
			FileOutputStream os = new FileOutputStream(outFile);
			workbook.write(os);
			os.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	
	public static XSSFCell cellAtAddr(XSSFWorkbook workbook, String cellAddr) {
		CellReference cr = new CellReference(cellAddr);
		XSSFRow row = workbook.getSheet(cr.getSheetName()).getRow(cr.getRow());
		if (row == null)
			row = workbook.getSheet(cr.getSheetName()).createRow(cr.getRow());
		XSSFCell cell = row.getCell(cr.getCol());
		if (cell == null)
			cell = row.createCell(cr.getCol());
		return cell;
	}
	
	public static void  evalCellSaveAtAddr(XSSFWorkbook workbook, XSSFCell evalCell, String destCellAddr) {
		FormulaEvaluator evaluator = workbook.getCreationHelper().createFormulaEvaluator();
		XSSFCell destCell = cellAtAddr(workbook, destCellAddr);
		evaluator.evaluateFormulaCell(evalCell);
		double dval = evalCell.getNumericCellValue();
		destCell.setCellValue(dval);
	}