Merge pull request #10 from calculquebec/rev/h26/chap2

Révision du chapitre 2

Author: plstonge

src/02-altair.ipynb

@@ -624,11 +624,14 @@
     "lang": "fr"
    },
    "source": [
-    "* **Colonnes temporaires** - Pour ajouter du bruit,\n",
-    "  on peut se créer des colonnes temporaires avec\n",
-    "  [`transform_calculate()`](https://altair-viz.github.io/user_guide/transform/calculate.html).\n",
-    "  Cependant, il faut spécifier explicitement\n",
-    "  [le type donnée](https://altair-viz.github.io/user_guide/encodings/index.html#encoding-data-types)."
+    "* **Colonnes temporaires** - Pour ajouter du bruit, on peut se créer\n",
+    "  des colonnes temporaires avec [la méthode\n",
+    "  `transform_calculate()`](https://altair-viz.github.io/user_guide/transform/calculate.html)\n",
+    "  * Le calcul des valeurs se fait au moyen d'une [expression\n",
+    "    Vega](https://vega.github.io/vega/docs/expressions/)\n",
+    "  * Pour accéder aux colonnes existantes, on passe par l'objet `datum`\n",
+    "  * À l'encodage d'un canal, on doit spécifier explicitement [le type\n",
+    "    de données](https://altair-viz.github.io/user_guide/encodings/index.html#encoding-data-types)"
    ]
   },
   {
@@ -638,11 +641,15 @@
     "lang": "en"
    },
    "source": [
-    "* **Temporary columns** - To add some noise,\n",
-    "  we can create temporary columns with\n",
-    "  [`transform_calculate()`](https://altair-viz.github.io/user_guide/transform/calculate.html).\n",
-    "  Once done, we have to specify\n",
-    "  [the data type](https://altair-viz.github.io/user_guide/encodings/index.html#encoding-data-types)."
+    "* **Temporary columns** - To add some noise, we can create temporary\n",
+    "  columns with [the `transform_calculate()`\n",
+    "  method](https://altair-viz.github.io/user_guide/transform/calculate.html)\n",
+    "  * The values are calculated using a [Vega\n",
+    "    expression](https://vega.github.io/vega/docs/expressions/)\n",
+    "  * To access the existing columns, we use the `datum` object\n",
+    "  * When encoding a channel, [the data\n",
+    "    type](https://altair-viz.github.io/user_guide/encodings/index.html#encoding-data-types)\n",
+    "    must be explicitly specified"
    ]
   },
   {
@@ -840,16 +847,18 @@
     "### Exercice - Créer un graphique à barres\n",
     "À partir du DataFrame `surveys_complet`, faites afficher le\n",
     "décompte du nombre d'enregistrements pour chaque `plot_id`. Instructions :\n",
-    "* Utilisez\n",
-    "  [`mark_bar()`](https://altair-viz.github.io/gallery/simple_bar_chart.html)\n",
-    "  pour générer\n",
-    "  [le graphique](https://altair-viz.github.io/gallery/simple_histogram.html)\n",
-    "* Sur l'axe horizontal, spécifiez la variable `'plot_id'` et le\n",
-    "  [type `'ordinal'`](https://altair-viz.github.io/user_guide/encodings/#encoding-data-types)\n",
-    "* Sur l'axe vertical, spécifiez `'count()'` comme variable pour que\n",
-    "  Altair fasse automatiquement le décompte, ce qui évite de passer\n",
-    "  par la méthode `groupby()` du DataFrame\n",
-    "* Activez le canal `tooltip` avec `'count()'` pour avoir le décompte\n",
+    "1. Utilisez [la méthode\n",
+    "   `mark_bar()`](https://altair-viz.github.io/gallery/simple_bar_chart.html)\n",
+    "   pour générer le graphique\n",
+    "   * Cette méthode donne accès à la fonction d'agrégation `'count()'`\n",
+    "     pour obtenir le décompte des enregistrements. Cela nous évite de\n",
+    "     devoir utiliser la méthode `groupby()` du DataFrame. [Voir un\n",
+    "     exemple ici](https://altair-viz.github.io/gallery/simple_histogram.html)\n",
+    "2. Pour l'axe horizontal, spécifiez la variable `'plot_id'` et le\n",
+    "   [type `'ordinal'`](https://altair-viz.github.io/user_guide/encodings/#encoding-data-types)\n",
+    "3. Pour l'axe vertical, spécifiez `'count()'`\n",
+    "4. Activez le canal `tooltip` pour voir le décompte au passage de la\n",
+    "   souris\n",
     "\n",
     "(6 min.)"
    ]
@@ -864,15 +873,18 @@
     "### Exercise - Create a bar chart\n",
     "From the `surveys_complete` DataFrame, create an histogram that\n",
     "shows the count of records for each `plot_id`. Instructions:\n",
-    "* Use\n",
-    "  [`mark_bar()`](https://altair-viz.github.io/gallery/simple_bar_chart.html)\n",
-    "  to generate the\n",
-    "  [chart](https://altair-viz.github.io/gallery/simple_histogram.html)\n",
-    "* For the X axis, specify the `'plot_id'` field and the\n",
-    "  [`'ordinal'` type](https://altair-viz.github.io/user_guide/encodings/#encoding-data-types)\n",
-    "* For the Y axis, specify `'count()'` as a temporary field computed\n",
-    "  automatically by Altair, which saves us from using `groupby()`\n",
-    "* Activate the `tooltip` channel with `'count()'`\n",
+    "1. Use the [`mark_bar()`\n",
+    "   method](https://altair-viz.github.io/gallery/simple_bar_chart.html)\n",
+    "   to generate the chart\n",
+    "   * This method provides access to the `'count()'` aggregation\n",
+    "     function to obtain the count of records. This saves us from using\n",
+    "     `groupby()`. [See an example\n",
+    "     here](https://altair-viz.github.io/gallery/simple_histogram.html)\n",
+    "2. For the X axis, specify the `'plot_id'` field and the [`'ordinal'`\n",
+    "   type](https://altair-viz.github.io/user_guide/encodings/#encoding-data-types)\n",
+    "3. For the Y axis, specify `'count()'`\n",
+    "4. Activate the `tooltip` channel to see the count when moving the\n",
+    "   mouse\n",
     "\n",
     "(6 min.)"
    ]
@@ -892,7 +904,9 @@
     "alt.Chart(surveys_complet).mark_bar().encode(\n",
     "    x=alt.X('plot_id').type('ordinal'),\n",
     "    y=alt.Y('count()'),\n",
-    "    tooltip=['count()'],\n",
+    "    tooltip=[\n",
+    "        alt.Tooltip('count()', title=\"Nombre d'enregistrements\")\n",
+    "    ],\n",
     ")"
    ]
   },
@@ -911,7 +925,9 @@
     "###(surveys_complet)###\n",
     "    ###('plot_id').type('ordinal'),\n",
     "    ###('count()'),\n",
-    "    ###['count()'],\n",
+    "    ###[\n",
+    "        alt.Tooltip('count()', title=\"Nombre d'enregistrements\")\n",
+    "    ],\n",
     ")"
    ]
   },
@@ -930,7 +946,9 @@
     "alt.Chart(surveys_complete).mark_bar().encode(\n",
     "    x=alt.X('plot_id').type('ordinal'),\n",
     "    y=alt.Y('count()'),\n",
-    "    tooltip=['count()'],\n",
+    "    tooltip=[\n",
+    "        alt.Tooltip('count()', title='Number of records')\n",
+    "    ],\n",
     ")"
    ]
   },
@@ -949,7 +967,9 @@
     "###(surveys_complete)###\n",
     "    ###('plot_id').type('ordinal'),\n",
     "    ###('count()'),\n",
-    "    ###['count()'],\n",
+    "    ###[\n",
+    "        alt.Tooltip('count()', title='Number of records')\n",
+    "    ],\n",
     ")"
    ]
   },
@@ -982,6 +1002,7 @@
     "        * Catégories discrètes : `'nominal'`, `'var:N'`\n",
     "        * Dates et heures : `'temporal'`, `'var:T'`\n",
     "    * `tooltip=['varX', 'varY', ...]`\n",
+    "      * `alt.Tooltip('count()', title=\"Nom de l'agrégation\")`\n",
     "* **Autres propriétés du graphique**\n",
     "  * `graphique.interactive()`\n",
     "* **Sauvegarde**\n",
@@ -1019,6 +1040,7 @@
     "        * Discrete unordered category: `'nominal'`, `'var:N'`\n",
     "        * Time or date value: `'temporal'`, `'var:T'`\n",
     "    * `tooltip=['field_name1', 'field_name2', 'field_name3', ...]`\n",
+    "      * `alt.Tooltip('count()', title='Aggregation name')`\n",
     "* **Other properties of the chart**\n",
     "  * `chart.interactive()`\n",
     "* **Saving the figure**\n",