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diff --git a/sys/gio/ggscale.x b/sys/gio/ggscale.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<gio.h>
+
+# GGSCALE -- Get the WCS scale in world coords per NDC at the point (x,y).
+# Used to convert offsets or sizes in NDC coordinates into world coordinates,
+# and vice versa.  If log scaling is in use we can only locally approximate
+# the scale.
+
+procedure ggscale (gp, x, y, dx, dy)
+
+pointer	gp			# graphics descriptor
+real	x, y			# point for which scale is desired
+real	dx, dy			# scale wcs/nds (output)
+
+pointer	w
+real	x1, x2, y1, y2, xs, ys, elog_scale
+real	log10e, elogr()
+data	log10e /0.434294482/
+
+begin
+	w = GP_WCSPTR(gp,GP_WCS(gp))
+
+	x1 = WCS_WX1(w)
+	x2 = WCS_WX2(w)
+	y1 = WCS_WY1(w)
+	y2 = WCS_WY2(w)
+	xs = WCS_SX2(w) - WCS_SX1(w)
+	ys = WCS_SY2(w) - WCS_SY1(w)
+
+	switch (WCS_XTRAN(w)) {
+	case LOG:
+	    dx = (x / log10e) * log10 (x2 / x1) / xs
+	case ELOG:
+	    # The following is an approximation.
+	    elog_scale = (elogr(x2) - elogr(x1)) / xs
+	    if (x < 10.0)
+		dx = (-x / log10e) * elog_scale
+	    else if (x > 10.0)
+		dx = (x / log10e) * elog_scale
+	    else
+		dx = (10. / log10e) * elog_scale
+	default:
+	    # LINEAR
+	    dx = (x2 - x1) / xs
+	}
+
+	switch (WCS_YTRAN(w)) {
+	case LOG:
+	    dy = (y / log10e) * log10 (y2 / y1) / ys
+	case ELOG:
+	    # The following is an approximation.
+	    elog_scale = (elogr(y2) - elogr(y1)) / ys
+	    if (y < 10.0)
+		dy = (-y / log10e) * elog_scale
+	    else if (y > 10.0)
+		dy = (y / log10e) * elog_scale
+	    else
+		dy = (10. / log10e) * elog_scale
+	default:
+	    # LINEAR
+	    dy = (y2 - y1) / ys
+	}
+end