Added plot style colors and some misc utils. See comments.

- Also exposed the sys crate's ImPlotRange, ImPlotLimits and ImPlotPoint
  to the user as well as changing the limit-setting interface to using
  those. I'm not super sure about doing this yet though, because it is
  more verbose for no real reason to the user and for the purposes of
  the abstraction it may be better to add facade types anyway.

- There is no protection against calling things like `is_hovered` from
  outside a plot. Will need to think about how to make this impossible
  by design - we could for example pass an object to Plot's closure
  that has these methods, and that object is not created anywhere
  else.

implot-examples/examples/hello_world.rs

@@ -1,5 +1,11 @@
 use imgui::*;
-use implot::{AxisFlags, Plot, PlotFlags, PlotLine, PlotText};
+use implot::{
+    get_plot_limits, get_plot_mouse_position, is_plot_hovered, pop_style_color, push_style_color,
+};
+use implot::{
+    AxisFlags, ImPlotLimits, ImPlotPoint, ImPlotRange, Plot, PlotColorElement, PlotFlags, PlotLine,
+    PlotText,
+};
 
 mod support;
 
@@ -20,13 +26,24 @@ fn main() {
                 ));
                 ui.checkbox(im_str!("Show demo"), &mut showing_demo);
 
+                // Create some containers for exfiltrating data from the closure below
+                let mut hover_pos: Option<ImPlotPoint> = None;
+                let mut plot_limits: Option<ImPlotLimits> = None;
+
                 // Draw a plot
+                push_style_color(&PlotColorElement::PLOT_BG, 1.0, 1.0, 1.0, 0.2);
                 Plot::new("Demo plot")
                     .size(400.0, 300.0)
                     .x_label("awesome x label")
                     .y_label("awesome y label")
-                    .x_limits(0.0, 6.0, Condition::FirstUseEver)
-                    .y_limits(-1.0, 3.0, Condition::FirstUseEver)
+                    .x_limits(&ImPlotRange { Min: 0.0, Max: 6.0 }, Condition::FirstUseEver)
+                    .y_limits(
+                        &ImPlotRange {
+                            Min: -1.0,
+                            Max: 3.0,
+                        },
+                        Condition::FirstUseEver,
+                    )
                     .with_plot_flags(&(PlotFlags::DEFAULT))
                     .with_y_axis_flags(&(AxisFlags::DEFAULT | AxisFlags::INVERT))
                     .build(|| {
@@ -43,7 +60,31 @@ fn main() {
                             .with_pixel_offset(10.0, 30.0)
                             .plot(2.0, 2.0, false);
                         PlotText::new("Vertical Text!").plot(0.1, 2.5, true);
+                        if is_plot_hovered() {
+                            hover_pos = Some(get_plot_mouse_position());
+                        }
+                        plot_limits = Some(get_plot_limits());
                     });
+
+                // Print some previously-exfiltrated info. This is because calling
+                // things like is_plot_hovered or get_plot_mouse_position() outside
+                // of an actual Plot is not allowed.
+                if let Some(pos) = hover_pos {
+                    ui.text(im_str!("hovered at {}, {}", pos.x, pos.y));
+                }
+                if let Some(limits) = plot_limits {
+                    ui.text(im_str!(
+                        "X limits are {:+10.3}, {:+10.3}",
+                        limits.X.Min,
+                        limits.X.Max
+                    ));
+                    ui.text(im_str!(
+                        "Y limits are {:+10.3}, {:+10.3}",
+                        limits.Y.Min,
+                        limits.Y.Max
+                    ));
+                }
+                pop_style_color(1);
             });
 
         if showing_demo {