Update -graticule command

REFERENCE.md

@@ -1,6 +1,6 @@
 # COMMAND REFERENCE
 
-This documentation applies to version 0.6.61 of mapshaper's command line program. Run `mapshaper -v` to check your version. For an introduction to the command line tool, read [this page](https://github.com/mbloch/mapshaper/wiki/Introduction-to-the-Command-Line-Tool) first.
+This documentation applies to version 0.6.62 of mapshaper's command line program. Run `mapshaper -v` to check your version. For an introduction to the command line tool, read [this page](https://github.com/mbloch/mapshaper/wiki/Introduction-to-the-Command-Line-Tool) first.
 
 ## Command line syntax
 
@@ -735,7 +735,7 @@ Create a graticule layer appropriate for a world map centered on longitude 0.
 
 `polygon` Create an polygon enclosing the entire area of the graticule. Useful for creating background or outline shapes for clipped projections, like Robinson or Stereographic.
 
-`interval=` Specify the spacing of graticule lines (in degrees). Options include:  5, 10, 15, 30, 45. Default is 10.
+`interval=` Specify the spacing of graticule lines (in degrees). Common options are: 5, 10, 15, 30, 45. Default is 10.
 
 ### -grid
 

src/cli/mapshaper-options.mjs

@@ -983,7 +983,7 @@ export function getOptionParser() {
   parser.command('graticule')
     .describe('create a graticule layer')
     .option('interval', {
-      describe: 'size of grid cells in degrees (options: 5 10 15 30 45, default is 10)',
+      describe: 'size of grid cells in degrees (default is 10)',
       type: 'number'
     })
     .option('polygon', {

src/commands/mapshaper-graticule.mjs

@@ -82,7 +82,9 @@ function addOutlineToGraticule(graticule, outline) {
 //
 function createGraticule(P, outlined, opts) {
   var interval = opts.interval || 10;
-  if (![5,10,15,20,30,45].includes(interval)) stop('Invalid interval:', interval);
+  if (Math.round(interval) != interval || interval > 0 === false) {
+    stop('Invalid interval:', interval);
+  }
   var lon0 = P.lam0 * 180 / Math.PI;
   var precision = interval > 10 ? 1 : 0.5; // degrees between each vertex
   var xstep = interval;
@@ -123,6 +125,7 @@ function createGraticule(P, outlined, opts) {
     return Math.abs(a - b) < interval / 5;
   }
 
+  // extended: meridian extends to pole
   function createMeridian(x, extended) {
     var y0 = ystep <= 15 ? ystep : 0;
     createMeridianPart(x, -90 + y0, 90 - y0);