[Backport 6.1] Confidence band on regression missed multipler of sigma

doc/examples/ex43/ex43.sh

@@ -14,7 +14,7 @@ gmt begin ex43
 
 	file=$(gmt which -G @bb_weights.txt)
 	gmt regress -Ey -Nw -i0:1+l $file > model.txt
-	gmt regress -Ey -Nw -i0:1+l $file -Fxmc -T-2/6/0.1 > rls_line.txt
+	gmt regress -Ey -Nw -i0:1+l $file -Fxmc -T-2/6/0.1 -C99 > rls_line.txt
 	gmt regress -Ey -N2 -i0:1+l $file -Fxm -T-2/6/2+n > ls_line.txt
 	grep -v '^>' model.txt > A.txt
 	grep -v '^#' $file > B.txt

doc/rst/source/gallery/ex43.rst

@@ -8,7 +8,7 @@ robust regression line using *reweighted least squares* and from
 this fit we are able to identify outliers with respect to the
 main trend.  The result shows dinosaurs were large and dumb,
 humans and some monkeys pretty smart, and the rest of the
-mammals doing alright.
+mammals doing alright. Band shows 99% confidence internal on regression.
 
 .. literalinclude:: /_verbatim/ex43.txt
    :language: bash

src/gmtregress.c

@@ -1335,8 +1335,8 @@ EXTERN_MSC int GMT_gmtregress (void *V_API, int mode, void *args) {
 								case 'r':	/* Residual */
 									out[col] = S->data[GMT_Y][row] - gmtregress_model (x[row], par);
 									break;
-								case 'c':	/* Model confidence limit (add x and y uncertainties in quadrature since uncorrelated) */
-									out[col] = t_scale * hypot (par[GMTREGRESS_SIGIC], par[GMTREGRESS_SIGSL] * fabs (x[row] - par[GMTREGRESS_XMEAN]));
+								case 'c':	/* Model confidence limit (add slope and intercept uncertainties in quadrature since uncorrelated) */
+									out[col] = t_scale * sqrt (par[GMTREGRESS_MISFT]) * hypot (par[GMTREGRESS_SIGIC], par[GMTREGRESS_SIGSL] * fabs (x[row] - par[GMTREGRESS_XMEAN]));
 									break;
 								case 'z':	/* Standardized residuals (z-scores) */
 									out[col] = z_score[row];