Is your feature request related to a problem? Please describe.

When building time-series charts (e.g., a timeline from past to future), there's no built-in way to highlight "Today" on the X-axis. Currently, users have to implement workarounds like:

• Using <ReferenceLine> with a custom label component
• Creating a custom tick renderer via XAxis tick prop
• Using the <Customized> component for fully custom SVG rendering

These solutions work, but they require boilerplate and don't integrate cleanly with the axis label system.

Describe the solution you'd like

A first-class way to add special markers/labels to specific axis positions. This could be:

1. A markers prop on XAxis/YAxis - Array of objects specifying value + label + optional styling

   <XAxis 
     dataKey="date" 
     markers={[{ value: Date.now(), label: "Today", style: { fill: "#ff6b6b" } }]}
   />

2. An <AxisMarker> component - Child of XAxis/YAxis for declarative markers

   <XAxis dataKey="date">
     <AxisMarker value={Date.now()} label="Today" />
   </XAxis>

3. Enhanced tick callback - Allow returning special styling/labels for specific tick values

Describe alternatives you've considered

• ReferenceLine with label: Works but the label floats above the chart area rather than integrating with the axis
• Custom tick component: Requires reimplementing all tick logic just to highlight one value
• Customized component: Maximum flexibility but requires manual SVG positioning and doesn't adapt to chart layout changes

Additional context

This is a common pattern in financial dashboards, project timelines, and analytics tools where distinguishing "now" from historical/projected data is important. Native support would make recharts more competitive with other charting libraries that support annotations out of the box.

Related: #1678 requests general annotations support, but this is specifically about axis-integrated markers.