Add Gantt support for 'today'

Feature request:
#2853

Author: bprb

docs/syntax/gantt.md

@@ -338,6 +338,71 @@ gantt
 
 > **Warning** > `millisecond` and `second` support was added in v10.3.0
 
+### Changing Today
+
+By default, the today marker uses the current date.  Use `today` to set a custom date, using the `dateFormat`.
+
+```mermaid-example
+gantt
+    title A Gantt Diagram With Custom Today
+    dateFormat YYYY-MM-DD
+    today 2024-01-20
+    section Section
+        A task          :a1, 2024-01-01, 30d
+        Another task    :after a1, 20d
+```
+
+```mermaid
+gantt
+    title A Gantt Diagram With Custom Today
+    dateFormat YYYY-MM-DD
+    today 2024-01-20
+    section Section
+        A task          :a1, 2024-01-01, 30d
+        Another task    :after a1, 20d
+```
+
+You can also use a duration:
+
+```mermaid-example
+---
+displayMode: compact
+---
+gantt
+    title A Gantt Diagram With Custom Today (Duration)
+    dateFormat  x
+    axisFormat  %L ms
+    today       18ms
+    todayMarker stroke-width:2px,stroke:#FF0,opacity:0.5
+
+    section Graphics
+    Draw 1:  a1, 0, 28ms
+    Draw 2:  after a1, 20ms
+
+    section Compute
+    Kernel:  b1, 20, 12ms
+```
+
+```mermaid
+---
+displayMode: compact
+---
+gantt
+    title A Gantt Diagram With Custom Today (Duration)
+    dateFormat  x
+    axisFormat  %L ms
+    today       18ms
+    todayMarker stroke-width:2px,stroke:#FF0,opacity:0.5
+
+    section Graphics
+    Draw 1:  a1, 0, 28ms
+    Draw 2:  after a1, 20ms
+
+    section Compute
+    Kernel:  b1, 20, 12ms
+```
+
+
 ## Output in compact mode
 
 The compact mode allows you to display multiple tasks in the same row. Compact mode can be enabled for a gantt chart by setting the display mode of the graph via preceding YAML settings.

packages/mermaid/src/diagrams/gantt/ganttDb.js

@@ -26,6 +26,7 @@ let dateFormat = '';
 let axisFormat = '';
 let tickInterval = undefined;
 let todayMarker = '';
+let today = '';
 let includes = [];
 let excludes = [];
 let links = new Map();
@@ -57,6 +58,7 @@ export const clear = function () {
   displayMode = '';
   tickInterval = undefined;
   todayMarker = '';
+  today = '';
   includes = [];
   excludes = [];
   inclusiveEndDates = false;
@@ -92,6 +94,14 @@ export const getTodayMarker = function () {
   return todayMarker;
 };
 
+export const setToday = function (txt) {
+  today = txt;
+};
+
+export const getToday = function () {
+  return today;
+};
+
 export const setDateFormat = function (txt) {
   dateFormat = txt;
 };
@@ -766,6 +776,8 @@ export default {
   getTickInterval,
   setTodayMarker,
   getTodayMarker,
+  setToday,
+  getToday,
   setAccTitle,
   getAccTitle,
   setDiagramTitle,

packages/mermaid/src/diagrams/gantt/ganttRenderer.js

@@ -756,7 +756,24 @@ export const draw = function (text, id, version, diagObj) {
     }
 
     const todayG = svg.append('g').attr('class', 'today');
-    const today = new Date();
+
+    let today = new Date();
+    const dateFormat = diagObj.db.getDateFormat();
+    const dbToday = diagObj.db.getToday();
+    let customToday = dayjs(dbToday, dateFormat.trim(), true);
+    if (customToday.isValid()) {
+      today = customToday.toDate();
+    } else {
+      const [durationValue, durationUnit] = diagObj.db.parseDuration(dbToday);
+      if (!Number.isNaN(durationValue)) {
+        // let dayjs do the math to support 'ms' and such
+        customToday = dayjs(0).add(durationValue, durationUnit);
+        if (customToday.isValid()) {
+          today = customToday;
+        }
+      }
+    }
+
     const todayLine = todayG.append('line');
 
     todayLine

packages/mermaid/src/diagrams/gantt/parser/gantt.jison

@@ -77,6 +77,7 @@ that id.
 "includes"\s[^#\n;]+            return 'includes';
 "excludes"\s[^#\n;]+            return 'excludes';
 "todayMarker"\s[^\n;]+          return 'todayMarker';
+"today"\s[^\n;]+                return 'today';
 weekday\s+monday                return 'weekday_monday'
 weekday\s+tuesday               return 'weekday_tuesday'
 weekday\s+wednesday             return 'weekday_wednesday'
@@ -144,6 +145,7 @@ statement
   | excludes {yy.setExcludes($1.substr(9));$$=$1.substr(9);}
   | includes {yy.setIncludes($1.substr(9));$$=$1.substr(9);}
   | todayMarker {yy.setTodayMarker($1.substr(12));$$=$1.substr(12);}
+  | today {yy.setToday($1.substr(6));$$=$1.substr(6);}
   | weekday
   | weekend
   | title {yy.setDiagramTitle($1.substr(6));$$=$1.substr(6);}

packages/mermaid/src/diagrams/gantt/parser/gantt.spec.js

@@ -49,6 +49,14 @@ describe('when parsing a gantt diagram it', function () {
     expect(parserFnConstructor(str)).not.toThrow();
     expect(ganttDb.setTodayMarker).toHaveBeenCalledWith('off');
   });
+  it('should handle a today definition', function () {
+    spyOn(ganttDb, 'setToday');
+    const str =
+      'gantt\ndateFormat yyyy-mm-dd\ntitle Adding gantt diagram functionality to mermaid\nexcludes weekdays 2019-02-01\ntoday 2019-02-04';
+
+    expect(parserFnConstructor(str)).not.toThrow();
+    expect(ganttDb.setToday).toHaveBeenCalledWith('2019-02-04');
+  });
   it('should handle a section definition', function () {
     const str =
       'gantt\n' +