fix #353 by adding more precision when displaying meridian grid labels

src/core/src/math/angle.rs

@@ -579,8 +579,10 @@ use std::fmt::Display;
 impl Display for Angle<f64> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self.fmt {
-            AngleFormatter::Sexagesimal { prec, plus, hours } => {
+            AngleFormatter::Sexagesimal { mut prec, plus, hours } => {
                 let unit = if hours {
+                    // to preserve the same angular precision the new prec is p+log10​(15) ≈ p+1.18 < p+2
+                    prec += 2;
                     self.to_hours()
                 } else {
                     self.to_degrees()