diff --git a/crates/chart/src/lib.rs b/crates/chart/src/lib.rs
index 4b68b8d17..f4e2895fe 100644
--- a/crates/chart/src/lib.rs
+++ b/crates/chart/src/lib.rs
@@ -26,7 +26,7 @@ use serde::{Deserialize, Serialize};
 pub use axis::{Scale, format_number, nice_scale};
 pub use raster::rasterize;
 pub use render::{Swatch, legend_entries, render, supports_trendline};
-pub use resolve::{default_palette, resolve, series_color, series_plot_kind};
+pub use resolve::{default_palette, has_volatile_series, resolve, series_color, series_plot_kind};
 pub use sparkline::render_sparkline;
 
 /// Straight (non-premultiplied) RGBA colour.
diff --git a/crates/chart/src/resolve.rs b/crates/chart/src/resolve.rs
index de3cdef1b..53dc177f1 100644
--- a/crates/chart/src/resolve.rs
+++ b/crates/chart/src/resolve.rs
@@ -104,6 +104,26 @@ fn eval_list_in(wb: &Workbook, sheet: usize, formula: &str, depth: u32) -> Vec<V
     }
 }
 
+/// Whether a series formula of `ch` calls a function whose result changes between evaluations
+/// (`NOW`, `TODAY`, `RAND`, `RANDBETWEEN`, `RANDARRAY`), so a result resolved earlier is stale.
+pub fn has_volatile_series(ch: &Chart) -> bool {
+    const VOLATILE: [&str; 5] = ["NOW", "TODAY", "RAND", "RANDBETWEEN", "RANDARRAY"];
+    let calls_volatile = |f: &str| {
+        // Formulas read from a file may carry the `_xlfn.` prefix of newer functions.
+        let up = f.to_ascii_uppercase().replace("_XLFN.", "");
+        VOLATILE.iter().any(|name| {
+            up.match_indices(name).any(|(i, _)| {
+                let before = up[..i].chars().next_back();
+                let after = up[i + name.len()..].trim_start();
+                before.is_none_or(|c| !(c.is_alphanumeric() || c == '_' || c == '.')) && after.starts_with('(')
+            })
+        })
+    };
+    ch.series.iter().any(|s| {
+        [s.name.as_deref(), s.categories.as_deref(), Some(s.values.as_str()), s.bubble_sizes.as_deref()].into_iter().flatten().any(calls_volatile)
+    })
+}
+
 fn to_num(v: &Value) -> Option<f64> {
     match v {
         Value::Number(n) if n.is_finite() => Some(*n),
diff --git a/crates/chart/src/tests.rs b/crates/chart/src/tests.rs
index 6e423802c..2e08f3d68 100644
--- a/crates/chart/src/tests.rs
+++ b/crates/chart/src/tests.rs
@@ -1330,3 +1330,32 @@ fn a_series_named_by_a_name_on_a_sheet_with_a_bang_resolves() {
     let d = resolve(&wb, 0, &c);
     assert_eq!(d.series.first().map(|s| s.name.as_str()), Some("Takings"));
 }
+
+#[test]
+fn volatile_series_formulas_are_detected() {
+    let with = |values: &str, name: Option<&str>| {
+        let mut c = chart(ChartKind::Line);
+        c.series.push(Series {
+            name: name.map(Into::into),
+            categories: None,
+            values: values.into(),
+            bubble_sizes: None,
+            color: None,
+            secondary: false,
+            kind: None,
+            smooth: false,
+            quartiles: Default::default(),
+            trendlines: vec![],
+        });
+        c
+    };
+    assert!(has_volatile_series(&with("=NOW()", None)));
+    assert!(has_volatile_series(&with("=SUM(A1:A3)*rand ()", None)));
+    assert!(has_volatile_series(&with("Sheet1!$B$2:$B$9", Some("=TODAY()"))));
+    assert!(has_volatile_series(&with("=RANDARRAY(5)", None)));
+    assert!(has_volatile_series(&with("=_xlfn.RANDARRAY(5)", None)));
+    assert!(!has_volatile_series(&with("Sheet1!$B$2:$B$9", Some("Sheet1!$B$1"))));
+    assert!(!has_volatile_series(&with("=MYRAND(A1)", None)));
+    assert!(!has_volatile_series(&with("=Sheet1!$NOW", None)));
+    assert!(!has_volatile_series(&chart(ChartKind::Line)));
+}
diff --git a/crates/ui-egui/src/app.rs b/crates/ui-egui/src/app.rs
index b67e37d99..535c91a5c 100644
--- a/crates/ui-egui/src/app.rs
+++ b/crates/ui-egui/src/app.rs
@@ -115,6 +115,7 @@ impl SheetApp {
         // Floating pictures' textures go with their workbook, even when no sheet is painted.
         let open: Vec<u64> = self.session.documents().iter().map(|d| d.uid).collect();
         floating_pictures::forget_closed(ctx, &open);
+        self.chart_cache.get_mut().unwrap_or_else(std::sync::PoisonError::into_inner).retain_docs(open.iter().copied());
         self.apply_font_state(ctx);
         if let Some(answer) = self.desktop_theme.take_change() {
             self.desktop_dark = Some(answer);
diff --git a/crates/ui-egui/src/chartcache.rs b/crates/ui-egui/src/chartcache.rs
index 2eb8510ad..82e9a96a2 100644
--- a/crates/ui-egui/src/chartcache.rs
+++ b/crates/ui-egui/src/chartcache.rs
@@ -461,4 +461,35 @@ mod tests {
         assert_eq!(h.state().chart_cache.lock().unwrap().docs(), 1);
         assert_eq!(n, 0, "the remaining document's charts were cached too");
     }
+
+    #[test]
+    fn a_chart_with_a_volatile_series_is_rendered_on_every_frame() {
+        let mut h = harness(two_charts());
+        {
+            let d = h.state_mut().session.active_mut().unwrap();
+            let sh = Arc::make_mut(&mut d.wb).sheet_mut(0).unwrap();
+            sh.charts[0].series[0].values = "=RAND()".into();
+        }
+        h.run_steps(4);
+        let n = rendered_after(&mut h, |h| h.run_steps(6));
+        assert!(n >= 6, "the volatile chart is resolved again on each frame, got {n}");
+        assert!(n < 12, "the other chart still comes from the cache, got {n}");
+    }
+
+    #[test]
+    fn closed_documents_are_forgotten_without_painting_a_chart() {
+        let mut h = harness(two_charts());
+        assert_eq!(h.state().chart_cache.lock().unwrap().docs(), 1);
+        exec(&mut h, "file.new", json!({}));
+        h.run_steps(4);
+        assert_eq!(h.state().session.documents().len(), 2);
+        // The charts' document is no longer shown, but still open.
+        assert_eq!(h.state().chart_cache.lock().unwrap().docs(), 1);
+        h.state_mut().session.set_active(0);
+        h.run_steps(2);
+        exec(&mut h, "file.close", json!({"force": true, "document": true}));
+        h.run_steps(2);
+        assert_eq!(h.state().session.documents().len(), 1);
+        assert_eq!(h.state().chart_cache.lock().unwrap().docs(), 0, "no chart is painted, yet the closed document's entries are gone");
+    }
 }
diff --git a/crates/ui-egui/src/chartview.rs b/crates/ui-egui/src/chartview.rs
index 0db26f4da..c769dbc1c 100644
--- a/crates/ui-egui/src/chartview.rs
+++ b/crates/ui-egui/src/chartview.rs
@@ -258,8 +258,11 @@ fn chart_prims(app: &SheetApp, p: &Painter, z: f32, wb: &Arc<Workbook>, si: usiz
     let Some(d) = app.session.active().filter(|d| Arc::ptr_eq(&d.wb, wb)) else {
         return render_chart(p, z, wb, si, ch).into();
     };
+    // A series that calls NOW or RAND must be evaluated again on every paint.
+    if gridcraft_chart::has_volatile_series(ch) {
+        return render_chart(p, z, wb, si, ch).into();
+    }
     let mut cache = app.chart_cache.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
-    cache.retain_docs(app.session.documents().iter().map(|d| d.uid));
     cache.get(d.uid, wb, si, ci, (z, p.ctx().pixels_per_point()), || render_chart(p, z, wb, si, ch))
 }