diff --git a/src/chart.ts b/src/chart.ts
index e07a5b3..9464621 100644
--- a/src/chart.ts
+++ b/src/chart.ts
@@ -513,7 +513,7 @@ export class Chart extends EventEmitter.EventEmitter {
         .enter()
         .append('path')
         .attr('class', 'y origin')
-        .attr('stroke', 'black')
+        .attr('stroke', 'currentColor')
         .attr('opacity', 0.2)
       yOrigin.merge(yOriginEnter).attr('d', this.line)
     }
@@ -533,7 +533,7 @@ export class Chart extends EventEmitter.EventEmitter {
         .enter()
         .append('path')
         .attr('class', 'x origin')
-        .attr('stroke', 'black')
+        .attr('stroke', 'currentColor')
         .attr('opacity', 0.2)
       xOrigin.merge(xOriginEnter).attr('d', this.line)
     }
@@ -674,12 +674,7 @@ export class Chart extends EventEmitter.EventEmitter {
       canvas.selectAll('.y.axis path, .y.axis line').attr('transform', 'translate(' + -xTranslation + ',0)')
     }
 
-    canvas
-      .selectAll('.axis path, .axis line')
-      .attr('fill', 'none')
-      .attr('stroke', 'black')
-      .attr('shape-rendering', 'crispedges')
-      .attr('opacity', 0.1)
+    canvas.selectAll('.axis path, .axis line').attr('stroke', 'currentColor').attr('opacity', 0.2)
   }
 
   private syncOptions() {
diff --git a/src/datum-defaults.test.ts b/src/datum-defaults.test.ts
new file mode 100644
index 0000000..db049c6
--- /dev/null
+++ b/src/datum-defaults.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, it } from '@jest/globals'
+import { FunctionPlotDatum } from './types.js'
+import datumDefaults from '../src/datum-defaults.js'
+
+describe('datumDefaults', () => {
+  it('should assign a random ID if one is not provided', () => {
+    const datum: FunctionPlotDatum = {}
+    const result = datumDefaults(datum)
+    expect(result.id).toBeDefined()
+  })
+
+  it('should default graphType to "interval" if not provided', () => {
+    const datum: FunctionPlotDatum = {}
+    const result = datumDefaults(datum)
+    expect(result.graphType).toBe('interval')
+  })
+
+  it('should default sampler to "builtIn" if graphType is not "interval"', () => {
+    const datum: FunctionPlotDatum = { graphType: 'polyline' }
+    const result = datumDefaults(datum)
+    expect(result.sampler).toBe('builtIn')
+  })
+
+  it('should default sampler to "interval" if graphType is "interval"', () => {
+    const datum: FunctionPlotDatum = { graphType: 'interval' }
+    const result = datumDefaults(datum)
+    expect(result.sampler).toBe('interval')
+  })
+
+  it('should default fnType to "linear" if graphType is "polyline", "interval", or "scatter"', () => {
+    let datum: FunctionPlotDatum = { graphType: 'polyline' }
+    let result = datumDefaults(datum)
+    expect(result.fnType).toBe('linear')
+
+    datum = { graphType: 'interval' }
+    result = datumDefaults(datum)
+    expect(result.fnType).toBe('linear')
+
+    datum = { graphType: 'scatter' }
+    result = datumDefaults(datum)
+    expect(result.fnType).toBe('linear')
+  })
+
+  it('should not modify the datum if all defaults are already set', () => {
+    const datum: FunctionPlotDatum = {
+      id: 'test',
+      graphType: 'polyline',
+      sampler: 'builtIn',
+      fnType: 'linear'
+    }
+    const result = datumDefaults(datum)
+    expect(result).toEqual(datum)
+  })
+})
diff --git a/src/samplers/interval.ts b/src/samplers/interval.ts
index 38dc012..a8bfc6c 100644
--- a/src/samplers/interval.ts
+++ b/src/samplers/interval.ts
@@ -12,7 +12,6 @@ import { SamplerParams, IntervalSamplerResult, IntervalSamplerResultGroup, Sampl
 
 async function asyncInterval1d({
   d,
-  xAxis,
   range,
   nSamples,
   nGroups,
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